They claim each other's waiver leftovers - not always, it seems, for a competitive advantage but rather in acts of gamesmanship.
The men in charge, Pete Carroll and Jim Harbaugh, have been fierce rivals dating to their college coaching days when they were trading words and wins in the Pac-10.
Here we go again, it's 49ers-Seahawks Part I this week.
"You go out and make plays, the chirping stops," 49ers quarterback Colin Kaepernick said Wednesday.
Everybody involved knows the NFC West will likely come down to these two nemeses when this season's all said and done, with a pair of dynamic young quarterbacks leading the way. The 49ers have captured the last two division titles, after Seattle won in 2010 with a losing record.
The QBs apparently have a friendly wager on this one, too. The loser must shave his eyebrow.
Not that Kaepernick would confirm his on-air bet with Russell Wilson, being hyped through EA SPORTS' Madden NFL 25 game.
"I'm going to let you ask Russell about that one," Kaepernick said. "Not my idea."
In the video clip, featuring the players sitting side by side in a locker room playing the video game, Wilson suggests the loser should shave his eyebrow, to which Kaepernick quips, "All right, I hope you don't like your eyebrow."
The Seahawks are miffed at Harbaugh for several things.
They claim he honked his horn at their team bus leaving Candlestick Park after San Francisco's win last October. Safety Earl Thomas insists that will only serve as fuel, along with all of the other jabs from both sides in recent months.
At least some Seattle players considered the move as mockery. Harbaugh has denied the honk, calling it a "fabrication."
"Of course we'd take it that way," Thomas said Wednesday. "When you carry yourself like a champion on and off the field, good stuff is going to happen for you. ... It just seems like he has a lot of personality, that's all fun and games. It's good for the sport of football. He's a competitor it seems like. Our coach is the same way. I don't think our coach would do that."
This budding rivalry between San Francisco and Seattle has intrigue on so many levels. And it appears to have staying power, too, as long as Carroll and Harbaugh keep winning and keep their jobs.
"They think they're the best, we think we're the best," 49ers running back Frank Gore said.
Carroll on Wednesday made several comparisons of the teams, from competitive spirit, to physical play to talent level. And how about coaching styles?
"What do you think? That's for you guys to figure out, not me," Carroll said.
The coaches certainly provide plenty of entertainment value.
"It is a fun thing to look at," Thomas said. "As a competitive standpoint, you definitely look at it as, OK, I know in their minds - they probably won't say it in front of their players - but they definitely want to win this game a lot more than other games. That just adds a cherry on top to the excitement building up to this game."
So far this week at least, Harbaugh is providing little new fuel ahead of Sunday night's NFC matchup at CenturyLink Field.
He even offered some kind words regarding Seahawks cornerback Richard Sherman and wideout Doug Baldwin, both former Stanford players. If there's any animosity on Harbaugh's part, he's taking the high road.
"Again, I don't know what you're talking about there," he said. "I have long and good memories, fond memories of both Richard and Doug when we were teammates. I understand now we're competitors. I wish them both long and healthy careers."
What Harbaugh said in June regarding performance-enhancing drugs and Seattle remains fresh for the Seahawks, however. Harbaugh took aim at Seattle's run of performance-enhancing drug infractions, which included an overturned suspension for Sherman.
"You always want to be above reproach, especially when you're good, because you don't want people to come back and say, 'They're winning because they're cheating,'" Harbaugh said then. "That's always going to be a knee-jerk reaction in my experience, ever since I was a little kid."
Go back even further, and there's been plenty of fuming by both sides.
Harbaugh's Cardinal traveled to Los Angeles in his first season at Stanford as 41-point underdogs only to stun the second-ranked Trojans 24-23 and end their 35-game home winning streak. It was largely considered the biggest upset in college football that year and among the best ever.
Then, in 2009, Harbaugh and No. 25 Stanford ran up the score on 11th-ranked USC in a surprising 55-21 rout, even attempting a 2-point conversion with the game way out of reach - prompting Carroll's infamous "What's your deal?" when they met afterward at midfield.
Now, there's a new deal - between Kaepernick and Wilson?
"I don't think Russell will be shaving his eyebrow then," Thomas said.
Gore said the Niners will do their part to keep all of Kaepernick's facial hair intact.
"That's the plan," Gore said.
Notes: The 49ers signed WR Kassim Osgood to a one-year contract to be a "core player" on special teams, and placed LB Nick Moody on the injured reserve/designated for return list. Harbaugh said he will miss seven games. ... San Francisco RB LaMichael James, nursing a sprained medial collateral ligament in his left knee, said he expects to practice Thursday and hopes to play Sunday. "I want to be out there," he said.