'NuMex Snowball' is a late maturing, intermediate-day, round onion that matures from July 25 to August 5 when spring seeded in Las Cruces, NM. The suggested planting dates at Las Cruces are January 15 to March 1. 'NuMex Snowball' has excellent yield, excellent white bulb color, excellent firmness, a moderate percentage of single centered bulbs, large bulbs, and a round shape. 'NuMex Snowball' originates from a population formed by intercrossing 'Ringmaster', a white selection from 'Texas Early Grano 502 PRR', 'Temprana', 'Snow White', and a pink root resistant selection from 'New Mexico White Grano' in 1980. Seed from each parent was kept separate and selections were made within each line using phenotypic recurrent selection for pink root resistance, white color, firmness, and bulb shape in 1981. Selections from each line were intercrossed and the resulting lines were kept separate. In 1981, 1983, and 1985, selections were made within each line and recombined after each selection. Selected bulbs from the third selection in 1985 were self-pollinated. Bulbs from the resulting progeny were selected (1987) within progeny families for relative bulb maturity. The latest maturing bulbs from 11 families were self-pollinated. Bulbs from the resulting progeny were selected for relative bulb maturity (1989) within progeny families. The latest maturing bulbs were recombined to form the next generation. Four cycles of phenotypic recurrent selection (1991, 1993, 1995, 1997) were completed to improve the population for bulb color, pink root resistance, maturity uniformity, bulb uniformity, bulb shape, firmness, and percentage of single centers.