Castor (Ricinus communis L.) is an industrial oilseed crop. Data on individual seed weight of castor seeds were taken as part of a study on adaptation of yield components to simulated levels of drought stress. It was found that when exposed to drought, castor seed weight does not follows a normal distribution, but a bimodal pattern. Light seeds (5 to 25% of the maximum seed weight) accounted for 11% of the seeds, while the number of medium weight seeds (26 to 55% of maximum seed weight) corresponded to only 3% of the seeds. Even under the most intense drought stress, the plant produces many seeds completely full. As more water is supplied to the plant, the frequency of light seeds is reduced. Light seeds have a normal embryo and tegument, but the storing tissues (endosperm) is not completely full. Fluctuations in individual seed weight seems to play an important role in the adaptation of the yield components to drought stresses. This change in relative seed weight would not be observed when only the mean seed weight is considered. Future research will be conducted on determining the germination capacity of light weight seeds, and if plants supplied with plenty of water supply would continue to produce light seeds.