Zigadenus nuttallii, a highly toxic spring ephemeral in tallgrass prairie, was studies in 1985 to ascertain: 1) several ecophysiological characteristics of the species, 2) seasonal patterns of biomass accumulation, and 3) its response to defoilation and fire. The maximum photosynthetic rate of Z. nuttallii measured in unburned prairie was 13.2 micromoles CO2 m-2s-1 which occurred at 24-28o C and an incident quantum flux of 0.8-1.0 mmoles m-2s-1. Maximum stomatal conductance measured was 5.4 mm s-1. Early in the season, belowground storage organs (bulbs) decreased in mass and supplied much of the energy for growth of leaves, even though CO2 uptake was possible. Bulb mass did not increase until about 6 weeks after shoot emergence implying that, at this time, leaves had become a source rather than a sink for carbohydrates. The result of a single, severe defoliation event was a decrease in biomass of bulbs, leaves and reproductive structures in Z. nuttallii. Intrinsic compensatory mechanisms were not detected. In contrast, fire, which also defoliated plants, did not result in any biomass decrease at the end of the season. Improved post-fire microclimate and increased nutrient supply (extrinsic factors) may have contributed to higher photosynthetic rates and led to biomass compensation in burned prairie. These data support arguments that intrinsic compensatory mechanisms have evolved in response to chronic herbivory. Key words: Photosynthesis, carbon storage, fire, herbivory, Zigadenus