The population biology of adult Monarch butterflies (Danaus plexippus) was studied during regular visits over three years (June–September 2013–15) at a milkweed (Asclepias speciosa)-rich site in central Washington. Small numbers of spring migrants colonized the site during June 5–17 each year and produced two adult generations one in early July and the other in late July–August, increasing the population at the site until mid–late August in 2013 and 2014. Greatest numbers of adults occurred in late July and August (20–24 per hour). In 2015 the population fell substantially in early August apparently as a consequence of heat wave conditions in late June–early July adversely affecting survival of second generation immature stages. Mark, release and recapture provided maximum population estimates at the site of 160–190 males, a recapture rate of 25–32% and intervals between tagging and recapture of 5–39 days. Sex ratio was imbalanced in favor of males on all dates ranging from 57–100%. Males patrolling milkweed patches was the most common behavior observed. Nectaring on A. speciosa and the exotic Purple Loosestrife (Lythrum salicaria) was frequently observed and the introduced Russian Olive (Elaeagnus angustifolia) was used for resting during the day. Dispersal from the site occurred in late August or early September. The use of large, dense areas of milkweed in relatively moist locations with some shade may be an important component of summer breeding of D. plexippus in the arid western United States. Expanding and/or creating additional such sites may be a useful conservation strategy for D. plexippus in the arid west.