This study was carried out to determine the ecological regions most suitable for soybean seed production in the State of Paraná, Brazil. At harvest, 1.226 seed samples of the early cultivar Paraná were collected, from different regions of the state, from the 1976 to the 1983. Growing seasons seed quality evalutions were: germination, blotter, testand tetrazolium test (to estimate field deterioration, mechanical damage, and sting bug attack). Three regions were previously defined in the state based on the average temperature (AT) in february: T1 region, with AT > 24°C; T2, with AT between 22 and 24°C; and T3, with AT < 22°C. Region T3 produced the best quality seed, followed by regions T2 and T1. Mechanical damage during harvest significantly reduced seed quality, especially in the 1976/77 and 1978/79 seasons. In general region T3 was the most favorable area for production of high quality seed in Paraná. Other factors such as mechanical damage, stink bug damage, and seed-borne pathogens were responsible for the lower seed quality of early soybean cultivars produced in the state of Paraná.