The seasonal occurrence of anaplasmosis (Anaplasma marginale) outbreaks in dairy and beef cattle was analysed for a region of Argentina (29 degrees to 31 degrees S and 58 degrees to 62 degrees W) that is free of Boophilus microplus ticks, using data collected from December 1978 to November 1995. The outbreaks were confirmed by inspection of blood smears obtained from sick or dead cattle. A total of 94 outbreaks were confirmed by inspection of blood smears obtained from sick or dead cattle. A total of 94 outbreaks were identified: 48 in beef cattle and 46 in dairy herds. The proportional seasonal distribution of outbreaks was different in beef and dairy cattle (chi-square = 15.08, P < 0.01). While no seasonal pattern of anaplasmosis outbreaks was found in dairy cattle, there was a concentration of outbreaks during the summer months (54% of the total) in beef cattle. Rural practices that are carried out more frequently on dairy than on beef cattle may have favoured iatrogenic transmission of A. marginale in the milk production system.