Over 20 000 seeds of the Washington lupin (Lupinus polyphyllus Lindl.) were examined and measured in an experiment carried out over a 10 year period (1989±1999). Four dierent groups of Washington lupin seeds were found: dark, patterned, grey and light seeds. During the 10 year experiment, the total average number of seeds per plant decreased from 2654 (1990) to 1220 (1999), there was a slight decrease in seed weight per plant and an increase in the average weight per seed. There was a clear seed size/number trade-o at the intraspeci®c level. The relative proportions of each seed group also changed with patterned seeds becoming dominant (50 % at the beginning and 90 % at the end of the experiment), grey seeds remained constant (constituting 10 % of the total seeds) and the proportion of both dark (33 % at the beginning and 5 % at the end) and light (10 % at the beginning and 5 % at the end of the experiment) seeds decreased. Six hundred and sixty nine seeds were found to have a dierent testa ornamentation; they were distributed among the dierent groups as follows: 48 % patterned seeds, 29 % dark seeds, 12 % grey seeds and 11 % light seeds. There were no signi®cant dierences in the physical dimensions of the ornamented seeds compared with all other seeds. The results suggest that the Washington lupin is a mixture of dierent species and botanical forms; this is discussed in relation to possible selection pressures to produce both smaller and larger seeds. The possibility that changes in testa ornamentation are in¯uenced by genes controlling the synthesis of the seed coat pigment is discussed.