Abstract: The photosensitive toxicity of α-terthienyl (α-T) on the hemocytes in Culex pipiens pallens larvae was under investigation. The results showed that α-T,in vivo and in vitro,obviously affected the morphological ductility and the proliferation of the four main hemocytes. α-T promoted the reproduction of hemocytes in a short time after processing,but then gradually inhibited the hemocytes. The inhibition enhanced as the concentration increased.Moreover,α-T showed different extent of photosensitive toxicity to the four hemocytes and strongly changed the proportion of the hemocytes.At the early period of processing,α-T promoted the reproduction of plasmalocyte and prohemocyte,while at the later period,α-T strongly decreased the proportion of granulocyte and spherulocytes.In addition,the promotion of α-T on the proliferation of the four hemocytes,in vitro,was significantly weaker than that in vivo,and lasted shorter time.