Phaeosphaeria leaf spot is a potentially important maize disease that has recently appeared in the continental United States in winter breeding nurseries in southern Florida. Inbred lines re lated to B73 are particularly susceptible to Phaeosphaeria leaf spot, whereas inbreds related to Mo17 are highly resistant. The inheritance of resistance to Phaeosphaeria leaf spot was studied in the parents, F1, F2, and backcross generations derived from the cross B73 × Mo17. A replicated experiment was conducted over two winter nursery seasons in a southern Florida nursery Individual plants in plots were evaluated for Phaeosphaeria leaf spot severity (0 to 9 scale) at the mid-dent stage of kernel development. Variation in segregating generations appeared continuous, so generation mean analysis was used. Additive and, to a lesser extent, dominance gene action were determined to play a role in the inheritance of reaction to Phaeosphaeria leaf spot There was no evidence for epistatic gene interactions. Heritabilities (both narrow and broad sense) were high (0.70 to 0.85) and the magnitude of genotype × environment interactions was low. Estimates of the number of effective factors (genes) involved in the inheritance of resistance ranged from three to four. Selection should be highly effective in developing inbred lines with adequate levels of resistance to Phaeosphaeria leaf spot.