The effect of inoculum density on the development of infection by Gigaspora decipiens Hall and Abbott, Glomus fasciculatum (Thaxter sensu Gerdemann) Gerdemann and Trappe and Glomus tenue Greenall (Hall) was investigated. From individual propagules, infection increased logarithmically for each fungus but, at higher inoculum levels, the infectivities differed. Of the three fungi, Gi. decipiens was the least, and Gl. fasciculatum the most infective. The fungi showed different strategies for infection spread. Glomus fasciculatum produced larger infection segments, and spread within the root more extensively than the other fungi. Gigaspora decipiens and GL tenue were more dependent on external spread by the formation of secondary entry points. The intensity of infections was affected by inoculum density, but not by the age of the infection.