A sex-linked recessive gene with visible effect will first be detected in the hemizygous sex (male). In lines with equal numbers of males and females, when the gene is initially present in a single female the probabifity of detection falls from 2/3 in single pair lines to 0·54 in large lines. The mean and standard deviation of time to detection are almost independent of population size, being about 4/3 and 2/3 respectively. About 98% of all detections occur within three generations, so a gene detected much later than this after the foundation of a selection line is likely to be a new mutant. Higher initial frequencies and selection favouring heterozygotes increase the chance of detection. The time taken is decreased with higher initial frequencies and increased slightly by selection favouring heterozygotes.