The genomic distribution of actin-related sequences in tomato was investigated using a cloned actin gene from soybean. Ten actin loci account for most of the hybridizing fragments observed with Southern analysis. Single loci were found on chromosomes 1, 3 and 10 and two loci on chromosome 4. One locus is linked to an unmapped isozyme marker, Sod-1. The four remaining actin loci are independent of each other and of any of the other markers tested. The number of actin loci in tomato (10) is greater than that estimated for soybean (8). As soybean is apparently a tetraploid and tomato a diploid, these results suggest that the number of actin loci has not been stable during the evolution of dicots. A number of these mapped loci lie in regions of the genome previously devoid of molecular markers and thus may be useful in basic and applied genetic research.