The effect of photoperiod on the haemolymph protein profile in the silkworm, Bombyx mori L. larvae was investigated at photoperiod treatments of light to dark (L:D) ratios of 13:11 (control), 24:0, 18:6, 12:12, 6:18 and 0:24 hours using fifth instar larvae. The intensity of protein bands in haemolymph extracts from larvae was measured with a densitometer. The intensity was highest in the haemolymph from larvae in the 24:0 (L:D) h photoperiod. Strongly reactive antigens of SP-1 and SP-2 and weakly reactive bands of storage proteins for both male and female larvae were observed in the 24:0 (L:D) h photoperiod.