Diel patterns of human host attack by Aedes albopictus in the laboratory were studied in relation to mosquito body size, age, parity, and time of day. Analysis of responses in 10-, 15-, and 20-day-old females indicated a significant main effect due to time of observation in the diel period, as well as significant time x parity and time x age interactions. The distribution of mean host attack responses during the diel period was bimodal with approximately 70% of all activity during photophase (0800-2000 h); attack rates were highest in the morning (0800 h) and evening (1400-2000 h) and lowest between 0200 and 0600 h. The diel pattern of attack responses was bimodal for nulliparous and parous females, but parous females were more active than nulliparous females between 1400 and 2000 h. This pattern became increasingly bimodal during photophase, as mosquitoes aged, regardless of mosquito body size or parity. Variations in host avidity patterns between young and old females suggest that mosquito repellent bioassays initiated early in the day, that last > or = 6 h, or that use young females (approximately 5 days old) overestimate the protection period of deet against mosquitoes > 10 days old.