Primary conidia of the entomopathogens Erynia (subgenus Neopandora) delphacis (1 isolate) and Erynia (Neopandora) neoaphidis (3 isolates) were stimulated to form germ-tubes with Tween 20 and with free, long-chain fatty acids, each incorporated into Entomophthora complete medium (ECM). When combined with other basal media (three tested), these compounds did not stimulate germ-tube formation. Triacylglycerols and vegetable oils, added to the same media, allowed almost complete resporulation in the fungi. In both species, Tween 20 (0.1%) encouraged greater germ-tube production (41-69%) than the fatty acids (0.1%) (< or = 36%). For E. delphacis, Tween 20 and the fatty acids differed significantly, but for E. neoaphidis the differences were almost always insignificant. Myristic and oleic acids stimulated germ-tube formation in both species. Palmitic acid allowed almost complete resporulation of the fungi, except for one isolate of E. neoaphidis that formed germ-tubes. Linoleic acid, tested only for E. delphacis, was fungistatic to most conidia. Higher concentrations of the fatty acids (< or = 1%) did not increase germ-tube formation, except 1% oleic acid which affected E. delphacis alone (>80% germination and germ-tubes). Linoleic acid, and sometimes also myristic and oleic, were fungistatic and/or toxic, depending on their concentration and on medium composition. Addition of fatty acids to ECM usually extended the lag period, and altered the morphology of the conidia and germ-tubes. These phenomena were not observed with Tween 20. Colonies were formed by E. delphacis alone, stimulated by ECM supplemented with Tween 20 or fatty acids. The results are discussed with respect to biological and physiological aspects of germination, and with respect to the mode of action of the fatty acids and the surfactant.