A small-colony variant (Vsm) of the primary form (Vp) of Photorhabdus luminescens MD from in vitro and in vivo cultures is described. Unlike the primary form, Vp, the Vsm variant is not the preferred diet of its nematode symbiont, a Heterorhabditis sp., does not support development and reproduction of the nematode, and is less pathogenic than Vp to Galleria mellonella larvae. Vsm cells were carried by 25% of infective juveniles, but they comprised a very low percentage (~0.4%) of the total cells carried by the juvenile. In vitro subculture and in vivo injection into the larvae with either Vp or Vsm always produced a mixture of both Vp and Vsm. In nematode-bacterium-infected G. mellonella larvae, the Vp population in the hemocoel was high (4 × 10⁹ to 5 × 10⁹ CFU/g of wet insect tissue) at 24 h after infection, decreased about 10-fold by 48 h, and then regained a high level at day 5 before decreasing at day 7 and then remaining relatively constant through day 15 postinfection. The Vsm population, under the same conditions as those of Vp, increased gradually to a high level (9 × 10⁸ CFU/g of wet insect tissue) at day 5 postinfection and then declined gradually through day 15.