The inheritance of resistance to neck rot, incited by Botrytis allii Munn, was studied in four crosses between resistant and susceptible bulb onion (Allium cepa L.) lines, one cross between two resistant lines, and one cross between two susceptible lines. Using tests of excised scale pieces inoculated with a spore suspension and incubated in plastic boxes at 20C for 6 days, parents, F1, F2, and backcrosses were assigned disease indices (DI) derived from infection severity scores of individual bulbs. Generation means and frequency distribution of DI indicated that inheritance was quantitative and mostly additive with a small amount of dominance for susceptibility. Estimates of gene effects using a three-parameter model also indicated that additive effects predominated, with some dominance and no epistasis. Broad-sense heritability estimates ranged from 42% to 63% in the resistant x susceptible crosses.