Using an antisense construct of the discoidin gene transfected into Dictyostelium, we have repressed the expression of the three endogenous discoidin genes. Transformants exhibit a greater than 90% reduction in accumulated discoidin mRNA and protein. Nuclear run-on assays show that both the endogenous and the antisense genes are transcribed. Since only minor amounts of endogenous gene transcripts and none from the antisense gene can be detected on blots, we suggest that hybrids are formed within the nucleus and are rapidly degraded. Discoidin is believed to play a role in cell-substratum interaction and exhibits homologies to fibronectin. Discoidin-minus mutants exhibit the developmental phenotype of not streaming on a plastic surface. Antisense transformants show a similar phenotype and are thus phenocopies of these mutants.