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Broom snakeweed, Gutierrezia sarothrae, is a weedy native shrub of the western rangeland. It is difficult and expensive to control by conventional means. Puccinia grindeliae has been identified as a potentially effective biocontrol agent. P. grindeliae is microcyclic in the field. Field-collected teliospores were 37.5-55.0 mum long by 17.5-28.8 mum wide, with a sidewall thickness of 2.5-5.0 mum and an apical wall thickness of 5.0-10.25 mum. The pedicel length of detached spores was 62.5-142.5 mum and pedicel width was 3.8-7.5 mum. Teliospore germination was favored by washing with sterile distilled water and was optimal at 15 C and 98.8% relative humidity. Under these conditions, teliospores germinated in 4-8 hr to form basidiospores, which germinated within 1 hr. Basidiospores produced in the laboratory from field teliospores were 12.5-15.0 mum long by 7.5-11.3 mum wide. Urediniospores from young pustules on plants inoculated to confirm Koch’s postulates were globose and 15.3-27.5 mum in diameter. Mature teliosori formed on plants 4-8 wk after inoculation. These results suggest that germination and infection of the host generally occurs during the evening and early morning hours in the desert Southwest.