Samples of South African kelp seaweeds collected over a period of 21 months were analysed for algin content (as a percentage of anhydrous mass) by a colorimetric method. This method was an adaptation of a procedure described by Blumenkrantz & Asboe-Hansen (1973), who used the colour reaction betweenmeta-hydroxydiphenyl and furfurol to measure concentrations of purified uronic acids and of uronic acids in urine. While the algin content may fluctuate as much as 50% within a year, there was little evidence to suggest any marked seasonal variation in the content of eitherMacrocystisorLaminariaor in stipe samples ofEcklonia. Results did suggest some seasonality in the algin content ofEckloniablades, with winter values up to 50% higher than summer values. However, this is much less than the seasonal fluctuations reported in blades of certain European kelps. The algin content ofLaminaria, and to a lesser extent ofEcklonia, was inversely related to size and class, and was found to vary within different regions of the plant. In general, blades ofEckloniaandLaminariacontained more algin than stipes, but there was little difference between stipe and blade samples ofMacrocystis.