The efficacy of transgenic, 2,4-D tolerant cotton, developed by CSIRO has been field tested over the past 2 seasons on an irrigated field at Premer, on the Liverpool Plains of NSW. The material tested was a single transgenic line in the Coker 315 cultivar containing an introduced 2,4-D degradation gene and it showed good tolerance to 2,4-D, even at high rates. The effects of spraying both transgenic and normal plants with 2,4-D were monitored in the 1996/97 season using visual damage symptoms and dry matter production. Dry matter cuts taken at the end of the season indicated complete tolerance of the transgenic line to 2,4-D at up to twice the normal field rate when applied during the vegetative growth stage. Good tolerance was also observed at flowering and boll-fill stages.