Efforts are being made to screen peanut (Arachis hypogaea L.) genotypes for resistance to invasion of seed by the Aspergillus flavus (Link ex Fries) group fungi and subsequent contamination by aflatoxins. Field inoculation studies are needed to develop reliable screening techniques that insure infection of susceptible peanut seed by the A. flavus group fungi and minimize the number of escapes. The objectives of this study were to evaluate the effects of A. parasiticus (Speare) inoculum formulation and timing of application on shell and seed colonization and aflatoxin contamination. Soil population densities of A. flavus and plant survival were also measured. Plots were inoculated using various rates either at-plant or at mid-bloom of an organic-matrix infested with A. parasiticus NRRL 2999, and/or using an aqueous conidial suspension of the fungus. Mid-bloom inoculation produced an increase of A. flavus group population densities in the soil. There were no statistical differences between treatments in aflatoxin content, or colonization of shells or seed. At-plant inoculation was associated with reduced plant emergence or survival. Inoculation at mid-bloom with the organic-matrix resulted in peanuts that ranked high in aflatoxin content. There were no correlations between aflatoxin content, colonization of seed or shells, and population densities of A. flavus in soil. Inoculation at mid-bloom with the organic-matrix, resulting in greater soil population densities and relatively higher aflatoxin levels, was an effective method to use in screening for resistance to invasion by A. flavus group fungi and contamination by aflatoxins.