A method is described for measuring the primary biodegradability of water soluble dyestuffs under anaerobic conditions and gives the results obtained on 22 dyes of commercial significance. The dyestuffs tested included mordant blue 13, mordant black 9, basic red 18, acid yellow 151, direct red 7, acid red 114, direct blue 15, direct yellow 12, reactive black 5, acid blue 113, direct black 19, direct black 22, reactive blue 19, acid blue 80, acid blue 25, basic blue 22, basic green 4, direct yellow 11, reactive blue 21, basic blue 3, acid orange 3, and basic yellow 28. The results for the 4 azo and 6 diazo dyes show that virtually all the participating laboratories found very substantial biodegradation, and bearing in mind that the inoculum source and precise experimental details were different in each lab, these results are in good agreement with each other. With the two poly-azo dyes the results for direct black 19 all showed a moderate to substantial degree of degradation, but with direct black 22 the spread of results was from 0 to 100 %. The four anthraquinone dyes showed results ranging from scarcely degraded in the case of acid blue 80, to substantial degradation for the other three. Of the remaining 6 miscellaneous dyestuffs no conclusions could be drawn with basic green 4. The other five all showed a moderate level of degradation. The results show that with the single exception of acid blue 80, all the dyestuffs tested can show a substantial degree of color removal and thus it seems a reasonable conclusion that the breakdown of dyestuff in the environment is likely to be initiated under anaerobic conditions. (Baker-IVI)