The seasonal occurrence of anaplasmosis (Anaplasma marginale) outbreaks in dairy and beef cattle was analysed for a region of Argentina (29° to 31° S and 58° to 62° W) that is free ofBoophilus microplusticks, 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 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 ofA. marginalein the milk production system.