The level of inhibition of germination of spores added to cell-free culture filtrates of Fusarium oxysporum Schlecht. ex Fr. declines with time. This decline can be accounted for bythe decreasing levels of volatile sporostatie factors and a stable vacuolation factor produced by F. oxysporum. After the disappearance of the volatile sporostatic factors, a stable principle is recognised in the culture filtrate by a persistent level of inhibition of spore germination which is not completely alleviated by the addition of nutrients. A stable inhibitor of spore germination has been isolated and characterised as nonanoic acid, and evidence is presented that this compound is widely produced among fungi and can inhibit the germination of spores of many fungal species. The contribution of nonanoic acid to sporostasis in cultures of/~. oxysporum is assessed. l~o~INSO~ and GA~V. TT (1969) reported the identification of eight volatile sporostatic factors produced in liquid cultures of several fungal species and commented on the possible contribution of these volatile substances to the inhibition of spore germination in cultures of F. oxy-8porum. The results indicated that cultures of this fungus might contain non-volatile sporostatic factors. This idea was also suggested by earlier work (ROBINSON et al., 1968) in which a semi-purified extract of the nonvolatile vacuolation factor was found to inhibit the germination of spores from the t4 different species of fungi used in the b~oassays. The vacuo-]ation factor has been found to be produced by many fungi (PARx and