Hypothesized relationships between milk composition and life history traits were examined by analyzing mid-lactation milks of seven lemurs (Eulemur fulvus, E. macaco, E. rubriventer, E. mongoz, Varecia variegata, Hapalemur griseus, Lemur catta), three bushbabies (Otolemur crassicaudatus, O. garnettii, Galago moholi), and two lorises (Nycticebus coucang, Loris tardigradus); partial data were also obtained for the lemuroid Cheirogaleus medius. There were no significant differences in milk composition among species within either Eulemur or Otolemur, but the four genera for which multiple samples were available (Eulemur, Varecia, Otolemur, and Nycticebus) exhibited large composition differences. Eulemur milk was, on average, very dilute (9.9% dry matter) and low in energy (0.49 kcal/g). These milks contained 0.9% fat, 1.2% protein, and 8.4% carbohydrate on a fresh weight basis. Protein energy comprised only about 15% of total milk energy. Varecia had significantly higher dry matter (13.5%), fat (3.2%), protein (4.2%), gross energy (0.80 kcal/g), and protein energy:total energy ratio (28%) than Eulemur. Milks of the lorisoid genera Otolemur and Nycticebus were very similar, and both had significantly higher dry matter (18.3, 16.3%), fat (7.6, 7.0%), and gross energy concentration (1.27, 1.13 kcal/g) than either lemuroid genus. Otolemur milk was higher in protein than Nycticebus milk. We conclude that lorises, bushbabies, and perhaps cheirogaleids produce relatively rich, energy-dense milks in comparison with anthropoid primates. However, dilute milks appear to be uniformly found among species of Eulemur and perhaps in Lemur catta. The milk of Varecia (and perhaps Hapalemur) is intermediate in composition. Differences in milk composition among prosimians may be related to differences in maternal care: prosimians that carry their young during lactation produce more dilute milks than do species which leave their young unattended for prolonged periods. When looking at primates as a whole, however, the picture is somewhat less clear, since the milks of some "parkers" like Varecia do closely resemble those of large anthropoid primates who carry their young. Am.