The present study was designed to find what seasonal effect mass selection would render when practiced within a maize (Zea mays L.) population. Three subpopulations of " Mezcla Varietales Amarillos" population (MVA) were selected for prolificacy and grain yield in three different seasonal conditions: (i) MVA-A selected in rainy seasons (A); (ii) MVA-B selected in dry seasons (B); (iii) MVA-AB selected under both seasonal conditions each year. Three cycles in MVA-A and-MVA-B, and six in MVA-AB were tested during three A and three B growing seasons for evaluation of progress.Mass selection in MVA-A resulted in an increase in grain yield of 10.5% per cycle when tested in the A seasons (direct response) and only 0.8% when tested in the B seasons (indirect response). For ears per plant the corresponding responses were 8.8 and 1.0%. The direct response in grain yield in MVA-B was only 2.5% per cycle whereas it was 7.6% when tested in the A seasons. The gain in ears per plant was 11.4% in the tests in the A seasons, but the direct response was 4.4% per cycle. In MVA-AB, there was a gain in grain yield of 5.3 and 1.1% per cycle in the tests in A and B seasons. For prolificacy, the respective gains were 7.0 and 3.3% per cycle.The authors acknowledge the cooperation of J. Antonio Rivera and L. Miguel Estrada, Agronomists, of the Corn Program of the Instituto Colombiano Agropecuario at Turipana Experimental Center for their invaluable help in collecting some of the data.This work was supported in part by grants from the Rockefeller Foundation to the corn program of ICA in Colombia and to the corn program at the University of Nebraska.