ABSTRACTSeasonal abundance and dormancy were examined in field-collected spores of the vesicular-arbuscular mycorrhizal fungus (VAMF)Gigaspora gigantea.Monthly collections of soil were made from a sand dune in Rhode Island from Aug, 1984, through Nov, 1985, andca.weekly collections were made from Aug, 1985, through Jan, 1986. Spore abundance (spores/100 cc soil) increased from July onward, and reached a maximum in Dec, indicating that the majority of spores were formed in late summer and fall. A Sept increase in spore abundance was part of a highly significant cubic trend. Percent germination of spores was highest (89–98%) from Dec to July and lowest (4–9%) in Sept and Oct. Decline in spore germination in Sept, 1984, and Sept, 1985, was highly significant, and germination between Aug, 1984, and Nov, 1985, was best described by a quartic equation. Spores collected in Sept, 1986 were dormant, but were released from dormancy by incubation at 5 C for 5 wk. Dormant spores stored at 20–25 C for 5 wk did not germinate. Approximate length of the dormancy period in the field was 2–9 wk. Monthly mycorrhizal inoculum potential (MIP) values of sand dune soil varied seasonally, ranging from 13.9% to 49%, with minimal VAMF colonization in Sept and Oct, similar to the spore germination pattern ofG. gigantea.