The grooming behavior of a group of brown howler monkeys was studied for one year in an Atlantic forest reserve of southeastern Brazil. A total of 290 grooming bouts were recorded and analyzed. The two adult females directed most of the grooming (91%), while the adult male was the major recipient (37%). Grooming between females, and between them and their siblings, also occurred quite often. On average, the group spent 2% of its daily time grooming, with a higher frequency around noon. There were significant differences, however, in time spent grooming between seasons; grooming was more abundant during the coldest seasons (autumn–winter) and rarer in hotter ones (spring–summer). A significant negative correlation was found between grooming time and temperature, but contrary to expectations, grooming time failed to correlate with both the group' diet and the demands of food-gathering, as measured by travelling time and day range length. A comparison of grooming behavior with other species of the genus suggests that brown, red (A. seniculus), and black howler monkeys (A. caraya) are more similar to each other than to mantled howlers (A. palliata), a result that probably is linked with the differing social structure and group size of the latter species. © 1995 Wiley-Liss, Inc.