Using autoradiography, we have determined cell cycle parameters in neuroblasts of III-rd instar larvae of D. melanogaster. The overall duration of the cell cycle is 9 h, tG1 = 4 h, tS = 3.5 h, tG2 = 1 h, and tM = = 0.5 h. Using histochemical staining for beta-galactosidase activity, we have determined the stage of expression of the reporter gene of P[1ArB] element inserted into chb gene. The chb gene, which codes for a centrosomal protein, is expressed during the last third of S phase. This result illustrates the feasibility of using the enhancer trap P[1ArB] to study differential expression of cell cycle genes.