Two morphologically indistinguishable races of Radopholus similis from Florida are presently identifiable only by host preference tests. One race attacks banana and other hosts, but not citrus. The other race attacks both citrus and banana, as well as other hosts. The two races normally reproduce by amphimixis. The morphology of the female reproductive systems of both races were examined and appeared to be identical. However, slight differences were observed in developmental stages of oogenesis between the two races. The two races were distinct with respect to karyotype. There were four chromosomes in the banana race and five chromosomes i n the citrus race. Karyotypic uniformity was found i n the two Florida populations of the citrus race.