We have examined directly the expression of one collagen gene (DCg1) during Drosophila melanogaster embryogenesis by means of in situ hybridization. Transcripts of this gene, which were demonstrated to encode a basement membrane type IV collagen chain, began to accumulate specifically in mesodermal derivatives at stages 12-13 of embryogenesis, and not before. Cells expressing this gene overlap, or are closely intermingled with, somatic and visceral mesoderm in stages 12-14. In stages 15-17, in addition to the strongly positive fat bodies, highly labelled cell spots are found scattered around all the parts of the gut and symmetrically on each side of the ventral nerve cord. They correspond to circulating mesodermal cells which we consider to be haemocytes or mesoblasts.