A study was conducted at two sites near the Universidad Autónoma Chapingo, State of Mexico, in 2001, in order to estimate the response in squash (Cucurbita pepo L.) to in situ participatory selection in terms of fruit quality and seed yield. Two native varieties from Sierra Norte de Puebla, selected in situ, were evaluated: Mazapa (selection cycles 1 to 3) and Libertad (selection cycles 1 to 3), plus a control, at a density of 27,639 plants·ha-1, in a randomized complete block design with four replications. The best response per selection cycle, based on averaging the two locations, occurred in the Mazapa variety for number of fruits per plant (0.29 fruits; 31.8 %), fruit height (1.0 cm; 6.1 %), fruit width (0.5 cm; 2.4 %), flesh thickness (0.1 cm; 5.8 %), seed height (0.034 cm; 1.6 %) and seed width (0.001 cm; 0.11 %). The Libertad variety was better in gain for seed weight per fruit (12 g·fruit-1; 21 %) and fruit weight per plant (0.1 kg·fruit-1; 6.6 %). The second selection cycle in the Mazapa variety had the best gain in fruit weight (3.77 kg), seed weight per plant (98 g), flesh thickness (2.6 cm), fruit height (23.6 cm), fruit width (20.3 cm), and seed width (0.934 cm). The third selection cycle in Mazapa showed the highest values for number of fruits per plant (1.49), fruit weight per hectare (123.5 t·ha-1), and seed yield per hectare (3.83 t·ha-1).