The modulating effect of feeding sea mustard (Undarina pinnatifida), a fiber-rich seaweed, during initiation and post-initiation phases of colon carcinogenesis was investigated in Sprague Dawley rats. Four groups of animals were exposed to the two weekly injections of a chemical carcinogen, azoxymethane (AOM). Animals were placed on diet containing 15% sea mustard during initiation. post-initiation or initiation+post-initiation phase of carcinogenesis for 10 weeks, and colonic aberrant crypt formation and cell proliferation were compared to those of rats fed semi-synthetic control diet. Results showed that sea mustard feeding significantly reduced the numbers of both aberrant crypts and aberrant crypt foci. Also, labeling indices and proliferation zones were significantly reduced in the colon of the rats fed sea mustard diets. These results indicate that the diet containing sea mustard may decrease the risk of colon cancer development, and a part of such effect may be mediated through both the blocking of initiation and the suppression of cell proliferation in the colonic crypts, although precise mechanisms should be further examined.