An abundant mycoflora was found on the seed of Abutilon theophrasti, Datura stramonium, Ipomoea hederacea and Polygonum pensylvanicum, which are colonizing annuals of early successional habitats. The fungal species differed among the plant species and most were specialists with respect to their hosts. A more extensive mycoflora occurred within seed of P. pensylvanicum and D. stramonium than within seed of I. hederacea and A. theophrasti. I. hederacea and A. theophrasti had greater resistance to fungal infection than P. pensylvanicum and D. stramonium. Fungi differed in their ability to penetrate seeds. Seed germination and seedling development were affected, both quantitatively and qualitatively, by the fungal isolates. P. pensylvanicum seedlings were more susceptible to fungal invasion than those of A. theophrasti or I. hederacea. Extracts from all tissues of I. hederacea and from the roots of A. theophrasti had antifungal activity. Little activity was detected in extracts from P. pensylvanicum. Plant densities of I. hederacea, D. stramonium and P. pensylvanicum remained fairly constant in the field, after seedling emergence, irrespective of density or species diversity. Fungi mainly affected seed germination and the development of young seedlings; at later stages, herbivory became more important in plant survival.