Sanya Richards-Ross, running less than 24 hours after winning the first U.S. gold in track and field, turned in the fastest 200 heat at 22.48.
Richards-Ross took it easy early, was fourth with about 50 metres left, then picked it up at the end.
Americans Carmelita Jeter, the silver medallist in the 100 on Saturday, and Allyson Felix, the runner-up in the 200 at each of the past two Olympics, also qualified. So did the other two Jamaicans in the field: two-time 100 champion Shelly-Ann Fraser Pryce and Sherone Simpson.
Toronto's Crystal Emmanuel also qualified with a time of 23.10 seconds.
The semifinals are Tuesday, the final Wednesday.