Genetic and cytological approaches have yielded significant insight into the mapping and organization of genes located in the heterochromatin of Drosophila melanogaster. To date, only a few of these genes have been molecularly characterized in detail, and their function unveiled. As a further step towards the identification of heterochromatic gene functions, we have carried out a cytological analysis of mitotic and meiotic cell divisions in mutants carrying different allelic combinations of l(2)41Aa, a gene located in the proximal heterochromatin of chromosome 2. Our results showed that larval brains of l(2)41Aa mutants display a high frequency of cells with irregularly condensed chromosomes. In addition, defective chromosome condensation was detected in male meiosis, consequently affecting chromosome segregation and giving rise to irregular spermatids. Taken together, these findings indicate that l(2)41Aa is a novel cell cycle gene required for proper chromosome condensation in both somatic and germ line cells.