The number of eggs and mobiles (immatures and adults) of Amblyseius manihoti on entire plants was determined during a 10-month period in a cassava field located in the Bahia region of Brazil. Observed percentages of eggs, immatures and adults were compared to theoretical stable age distributions (58 % for eggs, 24 % for immatures and 18 % for adults). These comparisons served as a basis to discuss the influence of fecundity, mortality and migration on the age structure of populations.