To elucidate the function of metazoan B-type lamins during development, new null mutations of the Drosophila B-type lamin gene, lamDm(0), were analyzed in parallel with the misg(sz18) mutation, a lamDm(0) allele reported previously. Although in all these mutants, lamin Dm(0) protein was undetectable in neuroblasts and imaginal disc cells from the second instar larval stage onward, cells continued to proliferate. In contrast to the embryonic lethality of another Drosophila lamDm(0) allele, lam(PM15), reported previously, lethality did not occur until late pupal stages. Chromosomal structure and the overall nuclear shape remained normal even at these late pupal stages, although obviously abnormal nuclear pore complex distribution was observed concomitant with the loss of lamin Dm(0) protein. Compensating expression of lamin C was not induced in the absence of lamin Dm(0). Thus, no lamin-containing nuclear structures were found in proliferating larval neuroblasts. We did find that developmental abnormalities appeared in specific organs during the late pupal stage, preceding lethality. Surprisingly, coordinated size increase (hypertrophy) of the ventriculus was observed accompanied by cell division and muscle layer formation. Hypertrophy of the ventriculus correlated with a decrease in ecdysteroid hormone receptor B1 (EcRB1) protein, and furthermore could be suppressed by a heat-inducible EcRB1 transgene. In contrast, both gonadal and CNS tissues exhibited underdevelopment.