The effect of litter size on behavioral development was investigated in 1384 offspring from 114 litters of CD-1 control mice. Litters were classified in four groups of size: 5 to 7 (I), 8 to 10 (II), 11 to 13 (III), and 14 to 17 (IV). Group III was regarded as the control group. The offspring were examined for behavioral development including surface righting at Postnatal Days (PND) 4 and 7, negative geotaxis at PNDs 4 and 7, cliff avoidance at PND 7, swimming behavior at PNDs 4 and 14, and olfactory orientation at PND 14. In behavioral development, surface righting at PND 7 was significantly affected in group I. Swimming direction at PND 4 was significantly affected in group IV, and those effects showed significant tendencies to be retarded as litter size increased. Other measured parameters showed no significant effect of litter size.