Growing season aridity and livestock grazing seasonality can influence biomass production of perennial grasses in dryland systems. For this study, we used a long-term dataset (1967-2004) to investigate the independent and joint effects of growing season aridity (De Martonne aridity index calculated for the months of June through September) and grazing season (yearlong continuous, fall, winter/spring, or summer season grazing) on the mean annual biomass (kg per hectare) of the perennial grasses Bouteloua eriopoda (black grama), Aristida spp. (threeawn), and Sporobolus spp. (dropseed) in a southwestern United States Chihuahuan Desert rangeland system. Biomass data were collected from 78 permanent sampling transects that were within one mile (1609.34 m) distance to water. Over the 37-year study period, total perennial grass biomass decreased as growing season aridity increased, but the extent of this relationship depended upon season of grazing and specific grass taxon. Aridity-related decreases in total perennial grass biomass were most severe in the summer and fall summer seasonal grazing pastures, primarily due to inherently low black grama levels. Our findings indicate that over time, summer and fall grazing can potentially exacerbate the negative effects of increasing aridity on perennial grass biomass.