The milk of two tropical species of Eared seals (Otariidae) was analysed. In the Galapagos fur seal (Arclocephalus galapagoensis), milk fat content decreased and calcium content increased with age of the young (Table 1), while protein content (12%) and potassium content (167 mg%) remained more or less constant. Fat content was 32% during the perinatal attendance period, later decreasing to 25%, while calcium increased from 57 to 70 mg%. The fewer samples of Galapagos sea lion (Zalophus californianus wollebaeki) milk contained on average 17% fat and 9% protein. Fat content in the milk of different otariid seal species correlates best with the average duration of maternal foraging trips and less with the weaning age of young (Fig. 1).