Freshly harvested Striga asiatica L. seeds will germinate in response to a stimulant only after the passage of time, an after-ripening period, and exposure to moisture at a suitable temperature, a conditioning period. To investigate the role of seed moisture content in the regulation of the after-ripening period, seeds were placed in chambers having specific relative humidity of 6%, 14%, 33%, 75% and 91% for 30, 60, 90 and 150 days. The seeds were then conditioned and germination percentage, response to tetrazolium and seed moisture contents were measured. Seeds at moisture contents less than 10% at the start of conditioning had germination of greater than 93%. Seeds at moisture contents over 10% at the start of conditioning could germinate between 60% and 3%, with germination decreasing as seed moisture content at the start of conditioning increased. The highest moisture content (17%) and lowest germination percentage (3%) occurred in seeds stored at 91% relative humidity for 150 days. There was a linear relationship of a high degree of correlation (0.997) between a positive tetrazolium test and germination capacity. Germination capacity of seeds could be changed from 90% to 3% by prolonged storage in water (dilute benomyl solution), causing `wet dormancy', then returned to 90% germination by returning to dry storage. Seed moisture content at the beginning of conditioning appears to control the responsiveness of the seeds to germination stimulants. The implications of these findings to the control of the parasite are discussed.