We compared pink bollworm (PHW), Pectinophora gossypiella (Saunders), oviposition on AET-5, a resistant breeding stock of cotton, Gossypium hirsutum L., with that on 'Deltapine 61' (DPL61), a susceptible cultivar. Our objective was to determine if the previously observed resistance of AET-5 was related to the egg-laying habits of the PBW moth. Distribution of eggs on the plant was similar under natural populations of insects in the field and under introduced populations (both in free-choice and no-choice experiments) in the greenhouse. Distributions were, however, different on the two cottons. Mean percentages of eggs on akils, bolls, and leaves, over environments, were 43, 39, and 137, respectively, of the total number of eggs on AET-5, and 53, 33, and 10%, respectively, of those an DPL-61. Total eggs/plant was significantly lower on @-5 than on DPL-61 in the field (218 vs. 331; mean of 2 tests) but not in the greenhouse (374 vs. 363; mean of 4 tests). The percentage of bolls/plant infested with larvae was lower in AET-5 in the field (28 vs. 36%) but higher in the greenhouse (53 vs. 46%). Number of PBW eggs/infested boll was little different in both environments. Entrance holes/infested boll for each egg laid was significantly lower on AET-5 only in the greenhouse (0.61 vs. 0.89) but slightly lower in the field (0.58 vs. 0.63). Number of insects/infested boll was significantly lower on AET-5 in both field (1.8 vs. 2.5) and greenhouse (1.4 vs. 2.5). Thus, the field resistance of AET-5 was caused by a lower rate of oviposition plus internal-boll antibiosis after the larvae penetrated the boll. The lower rate of oviposition on AET-5 in the field probably was caused by its earlier flowering and fruiting than that of DPL-61, coupled with the late-season buildup of BPW populations. In the greenhouse, this difference disappeared probably because the two cottons were at a similar growth stage, i.a., early flowering and fruiting, when large numbers of moths were laying eggs on the plants. Resistance of AFT-5 in the greenhouse was caused by internal-boll antibiosis, as in the field, plus fewer entrance holes/boll for each egg laid, an effect that was expressed only weakly in the field.