Adult mayflies (Ephemeroptera) were sampled using sticky traps at several locations along a Rocky Mountain stream. Eight genera were collected, of which imagosoiBaetisspp andRhilhrogena hageniwere most common.Baetiswas represented almost exclusively by females and flight direction was significantly in the upstream direction. InR. hageniboth males and females were included. Females appeared to fly into clearings or downstream, whereas males showed no directionality, or a downstream bias. This difference in adult flight direction may be partly explainable by the greater amount of downstream drift inBaetisrelative toR. hageni.Finally, mermithid nematodes were found to be frequent parasites ofBaetisbut not ofR. hagenicausing total absence of eggs in infected females.