Artificial and natural infestation methods were used to separate internal-boll antibiosis from other types of resistance in cotton, Gossypium hirsutum L., to the pink boll worm (PBW), Pectinophora gossypiella (Saunders). Nine cottons, which included seven Texas race stocks (T-) and a susceptible and resistant check, were planted in field plots at Phoenix, Ariz., in 1979. Green bolls, 14 to 21 days old, in one row of each plot were infested weekly with PBW eggs on 1-cm2 slips of paper. Green bolls were harvested from half of the row and seed cotton was harvested from the other half. One race stock, T-53, had significantly fewer entrance holes per boll than the susceptible check, ‘Deltapine 61’ (DPL-61), while T-39, T-40, T-53, and T-167 had significantly fewer insects per boll. T-39 and T-705 caused lower insect survival in the boll—i.e., lower ratios of insects per boll to entrance holes per boll—than did the susceptible check. Seed that was X-rayed showed that T-39, T-167, and T-705 had significantly lower percentages of seed damage than DPL-61. In naturally infested plots, only T-39 and the resistant check, AET-5, had less seed damage than the susceptible check. Subsequent tests suggested that T-39 contained both PBW-resistant and PBW-susceptible component lines. In 1982, in plots infested artificially via a repeating pipette method, only 1 of 10 progenies of selected T-39 sublines had significantly less seed damage, but seven sublines had significantly more, than DPL-61. In 1983, two T-39 sublines, selected for low damage in 1982, had significantly less damage than DPL-61. The one T-39 subline selected for high damage in 1982 did not differ significantly in damage from DPL-61 in 1983. T-167, the other race stock selected from the 1979 test, failed to show resistance in the 1983 test. Five (AET-5 × T-167)F4 hybrids had significantly less seed damage than T-167 but did not differ from AET-5 in the 1983 test.