This study of territorial behavior in the nymphalid butterfly Vanessa atalanta examines a number of behavioral and climatic factors shaping how, when, and for how long males maintain territories during the late afternoons of late spring and summer. Territorial V. atalanta males were observed on the Iowa State University campus from early April/early May to late August/early September from 1987 to 1992 on a total of 224 days. Microclimatic conditions were simultaneously recorded with automatic datalogging equipment. Males started territorial activity later on warmer days and when the ground was cooler, under all types of cloud cover. Although body temperature depends most strongly upon solar radiation, ambient and surface temperatures were more strongly correlated with territorial start time. V. atalanta males therefore do not always start territorial behavior at times when they can reach optimal body temperatures by dorsal basking;The time of end dance, the last interaction between a V. atalanta territory occupant and an intruder, correlated with both ambient temperature and interaction frequency during the territorial period. Vagrant V. atalanta males stopped flying later on warmer days, and when mean interaction frequency was higher, and when wind velocity was higher. Flying V. atalanta males were limited by a lack of radiant energy on cooler days. On warmer days, however, oncoming darkness at twilight, rather than lack of radiant heat, seemed to limit the activity period;End time, when V. atalanta males leave their territories, correlated most strongly with end dance time. Butterflies also leave later on warmer days, probably due to the fact that end dance time also varies with T[subscript] a. Occupants also leave later when interaction frequency is higher. Butterflies leave considerably earlier on cloudy days than they do on sunny ones. This may be related to either surface temperature or diffuse solar radiation levels, or both. These three studies together yielded a climate space model showing the territorial activity period under various conditions;Behavioral and climatic factors affecting outcome and duration of territorial interactions were also examined. Larger occupants which chased during an interaction were more likely to win. Occupants also tend slightly to lose their territories to intruders more often when the air temperature is warmer. Interactions are shorter on warmer days and when the two contestants are more disparate in size. Strategic factors more strongly affected whether a butterfly won, lost, or tied in an interaction than did climatic factors such as ambient temperature.