SUMMARYThe yearly incidence of root colonization and extramatrical spores of vesicular-arbuscular (-VA) mycorrhizal fungi were determined for six agronomic crops grown in monoculture for 7 yr on a newly cleared woodland site in northwest Florida. Thirteen species of VA-mycorrhizal fungi were identified from the test site from 1972 to 1978. Sorghum yielded the greatest number of species (12) from a single crop during this period. The highest number of spores was associated with soybean and the lowest number of spores was found in the native woodland. Spores ofGigaspora margarita, G. gregaria, andG. giganteawere most numerous from soil around soybeans whileGlomus fasciculatusandG. claruswere most numerous around roots of bahia grass.Acaulosporaspp. were most abundant from soil around cotton and peanut. The yearly incidence ofGigaspora margaritaspores increased whileG. gregariaandG. giganteadecreased in numbers during the 7-yr study.Glomus macrocarpusvar.geosporusandG. fasciculatuswere observed in 1971 and/or 1972 but were not recovered again until 1978. Only two species,Gigaspora margaritaandG. gregaria, were recovered each yr that samples were taken. The highest level of root colonization occurred in 1972 and was followed by a decline in 1973–1974, but mycorrhizae showed a general increase in 1975–1976. It is suggested that the observed changes in the incidence of VA-mycorrhizal fungi were primarily due to the agricultural system of monoculture.