Summary Daily and hourly variation in numbers at water and reproductive behaviour of the damselfly, Hetaerina americana were studied under field conditions. Numbers were very similar near noon on 38 days and at each hour on 2 days when hourly counts were obtained. Males probably spent the night at water. Unpaired females were routinely present at the aquatic habitat, and indications of territorial activity in this sex were obtained. Neither sex displayed. Males were inattentive to motionless females but approached them sexually when they flew. Sperm transfer followed each observed seizure, averaged only 7 sec, and occurred while the male held the female in tandem. Copulation quickly followed sperm transfer, averaged 3 min, and was at water. After copulation, pairs flew briefly in tandem, separated without distinctive behaviour, and the female descended to oviposit unaccompanied in willow roots for an average of 27 min. Underwater oviposition seemed obligatory. Each female remained near the site of descent while her mate remained nearby throughout her submergence and exhibited guarding behaviour by flying toward and displacing conspecific male intruders.