Nesting biology and phenology in an aggregation of the primitively eusocial ground-nesting bee Halictus farinosus were studied at Green Canyon, Utah from May to August, 2010. Nest architecture was typical of the genus. Nests were small with an average of 3.5 worker and 13.5 reproductive brood per colony. Most workers were mated (77.5%) and had ovarian development (73.4%). The queen-worker size differential was moderate (8.8% for head width and 6.2% for wing length), indicating that sociality in this species is of intermediate strength compared to other social Halictus species. Results from 2010 were compared with those from 1977/1978 and 2002. Varying weather patterns in the years of study led to changes in phenological milestones: in the colder and wetter spring of 2010, nesting behavior was delayed by up to two weeks compared to the other years. While nest productivity was comparable among years, in 2010 the size difference between queens and workers was significantly larger than in 2002, indicating an effect of annual variation in weather conditions on social parameters in this species.