Pseudomonas fluorescens and a bacterium with similar characteristics (isolate A) were obtained from females of Heterodera avenae which caused growth-inhibition 'halos' in colonies of Verticillium suchlasporium or Acremonium sp. from naturally infected young females of H. avenae. These bacteria partially inhibited the growth of the egg-parasites Paecilomyces carneus and Acremonium sp. on agar. Isolate A also inhibited the growth of V. suchlasporium while Cladosporium cucumerinum, a fungal plant pathogen, was unaffected indicating a certain degree of specificity in the mycostasis. The percentage of nematodes infected by nematophagous fungi belonging to the genus Verticillium was less in young females than for newly formed cysts. The young females may have been initially protected by mycostasis which gradually decreased allowing the Verticillium spp. to infect the newly-formed cysts. The importance of mycostasis in the understanding of the process by which H. avenae is infected by nematophagous fungi under field conditions has still to be investigated.