We have previously demonstrated effective inhibition of Dictyostelium discoideum myosin heavy chain (MIIHC) gene expression after introduction of an antisense gene on a transformation vector into cells. We now report that transcription of a sense orientation MIIHC gene has a similar, though not as dramatic, effect on expression from the endogenous gene. Cells transformed with a sense fragment of the MIIHC gene are defective in cytokinesis and become giant and multinucleated when growing on a surface. This morphological phenotype is correlated with decreased expression of the endogenous MIIHC gene. Sense transformants accumulate less MIIHC protein and mRNA than control transformed cells, concomitant with accumulation of a vector-derived sense transcript. Expression of the introduced sequences seems to be required for the sense effect. The possible causes of this surprising phenomenon are discussed.