Fibreoptic endoscopy was performed in 1056 monomorphic birds of 144 different species belonging to 15 Orders. The purpose was to establish their sex by direct inspection of the gonads. The results suggest that although the technique involves surgical intervention it is generally more practical and accurate than chromosome analysis of lymphocytes or analysis of faecal steroids. Surgical anaesthesia is recommended on practical and humane grounds but this and the technique itself can be hazardous in old and fat birds, where a mortality rate of as high as 2 per cent was encountered. Cloacal examination of as high as 2 per cent was encountered. Cloacal examination is a more practical technique for sex determination in penguins.