Evidence is presented that in the true slime mould Physarumpolycephalum the ability of the amoebal strain CLd-AXE to grow in axenic medium is determined by mutations in two genes axeA and axeB and that the axenic growth of RSD~-AXE amoebae is also under genetic control. Mutant amoebal strains are also able to grow on bacterial lawns and the ability to grow in axenic media persists during prolonged culture on bacteria. However, some of the mutant strains grow less well on bacteria than strains of similar genetic background which are unable to grow in axenic media. CLd-AXE has the same nuclear DNA content (0.59 pg per nucleus) as a d , the strain from which it was derived. Amoebae able to grow in axenic media were also derived from strains ~2 7 and ~~8 5 8 which carry mutations for temperature sensitivity and leucine requirement expressed in the plasmodia1 phase. Tests in axenic media showed that these mutations were expressed in the amoebal phase. The elucidation of the genetic basis of axenic growth will allow the construction of a range of amoebal strains able to grow in axenic media and this will greatly facilitate the isolation and analysis of mutants in this organism.