ABSTRACT Defects in the locus Egfr, encoding the Drosophila EG F receptor homologue (DER), affect the development of the Malpighian tubules. They form as much shorter structures than in wild-type embryos, containing a reduced number of cells. The severity of this phenotype in seven alleles that we have analysed correlates with other embryonic defects caused by Egfr mutations. Interestingly the two pairs of tubules arc affected with different severity, with a greater reduction in cell number in the posterior pair than in the anterior. Temperature shift experiments indicate a role for this receptor in the regulation of tubule cell division. We also suggest that an additional role for DER in the allocation of cells to the tubule primordio is possible.