Ammonia has been shown to inhibit germination of conidia of several fungi in concentrations apparent in some soils. Formation of resting structures as well as germination can be considered as differentiation processes; therefore the possible role of ammonia in chlamydospore formation was investigated. In 25 ml erlenmeyer flasks containing lOmlTris-HCl (50 mM) + 0.05% MgCl2(pH 7.5), pregerminated conidia of several isolates of Fusarium oxysporum f. sp. dianthi and Fusarium oxysporum f. sp. raphani produced chlamydospores abundantly within 8 days. Addition of 0.05, 0.5 or 5 mM NF14C1 did not influence the amount of chlamydospores, but somewhat retarded their formation. High concentrations of NH4C1 (50 or 500 mM) reduced the amount of chlamydospores, but this effect could be ascribed to the chloride ions or the high molarity. To exclude a possible nutrional effect of NH4CI masking an inhibition of chlamydospore formation, the non-metabolisable analogues methylamine and trimethylamine were tested. The results were the same as with NH4C1. Chlamydospores formed in the presence of different concentrations of NH4CI were equal in size and morphology to those of the control. After 8 weeks there was no variation in persistence and viability of these chlamydospores buried in soil.