Responses to treatment with the herbicide dalapon have been determined and the karyotypes studied in four chromosome races of Cynodon dactylon. No relationship was found between tolerance to dalapon and the level of ploidy. At the dalapon concentration where differences in response were most apparent (2-5 mM), the difference in mean value of the two diploid clones was greater than any other pair of values. No positive correlation was found between tolerance and the morphology of the leaf surface. Karyotype studies have not resolved the conflicting evidence relating to the origin and inter-relationships of the chromosome races of this species.