'NuMex Chaco' is a short-day, grano-type onion that matures from May 20 to May 30 when fall seeded in Las Cruces, NM. Suggested planting dates at Las Cruces are September 20 to October 1. 'NuMex Chaco' has excellent yield, firmness, pink root resistance, bolting resistance, high percentage of single centered bulbs, round to slightly top shape, good scale quality, and early maturity, similar to 'NuMex Mesa'. 'NuMex Chaco' produces a higher percentage of single centered bulbs than any other cultivar in its maturity class. 'NuMex Chaco' is recommended for fall-seeding to provide a harvest of highly single centered bulbs for ring processing or fresh market during late May. 'NuMex Chaco' originates from a 'Texas Early Grano 502' pink-root-resistant population ('TEG 502 PRR') that was selected for bolting resistance. This bolting-resistant selection of 'TEG 502 PRR' was crossed as the female parent with 'Southport White Globe' in 1980. Seeds were harvested from 'TEG 502 PRR' and planted the following year. White bulbs with good pink root resistance, bolting resistance, bulb shape, firmness, and good color were selected in 1981. The selected bulbs were intercrossed and three additional selections in 1983, 1985, and 1987 were made for yellow bulbs, bolting resistance, pink root resistance, bulb shape, uniformity, and maturity using phenotypic recurrent selection. Yellow bulbs from the third selection were grouped to make sib-crosses among three bulbs. The resulting progeny were screened for segregation of yellow and white bulbs. Six families that produced only yellow bulbs were selected and recombined in 1989. In 1991, 1993, 1995, and 1997, selections were made using recurrent selection and each year's selections were intercrossed to form the next generation. The selection criteria were the same as mentioned previously except that the last three generations were also selected for the production of single growing points per bulb (single center).