Infective juveniles (IJs) of an entomopathogenic nematode strain native to southern Mexico transported an associated bacterium that turned the infected Galleria mellonella (L) larvae reddish. The nematode isolate (LBIN-1) was identified as Steinernema carpocapsae (Filipjev) by ITS sequence, and its associated bacterium (LBSe-17) as Serratia marcescens Bizio by 16S rDNA sequencing. Infectivity of S. marcescens LBSe-17 was confirmed by following Koch's postulates on G. mellonella larvae. Phoresis of the associated S. marcescens bacterium by the nematode into the G. mellonella larvae was corroborated by exposing G. mellonella larvae to S. marcescens either alone or mixed with the nematode. No larval mortality was observed in the first treatment, while 100% mortality was observed in the second treatment. S. marcescens was superficially carried by the IJs, as confirmed by surface sterilization of IJs, which caused total larval mortality but no growth of S. marcescens. Artificial induction of a similar association was achieved by mixing the S. marcescens strain with another Steinernema sp. strain (LBIN-2), showing total larval mortality of G. mellonella larvae and proliferation of S. marcescens in the cadavers. However, 5 consecutive cycles of larval infections showed that colony forming unit (CFU) counts from the larval cadavers declined sharply in the artificially induced association, while the natural association LBIN-1/ LBSe-17 declined, but then tended to stabilize.