Microcebus murinus is a photoperiod-dependent polyoestrous seasonal prosimian. In captivity, ♀♀ kept isolated or paired with a ♂ undergo 3 oestruses (April, June, August) during the breeding season. When 2, 3 or 4 pairs are kept together, a 25 % decrease in occurrence of the 2nd and 3rd seasonal oestrus is noticed. Life in social groups induces: — resorption or abortion in about 50 % of grouped ♀♀, — a high infant mortality (50 % of newborn young), — a significant modification of sex ratio (excess of ♂♂). Finally, the body weight gain is significantly reduced during the first two months of life in offspring of grouped ♀♀.