Phenotypic and genetic parameters in a commercial Merino flock. A study was made of a commercial Merino flock on the farm Rietfontein near Soutpan in the central Orange Free State . The progeny of 43 rams and 542 ewes were available for measurement at weaning. Measurements at other ages were done on 489 individuals. Only sex had a significant (P < 0,05) influence on most of the production traits. Genetic parameters were calculated by half-sib analysis. The heritability of bodymass at the age of 6( 0,41), 11 (0,41), and 15 months (0,46) seemed to be high enough to facilitate individual selection. A relatively low value was obtained with regard to the 15 – 16 months greasy-fleece mass and also for greasy-fleece mass (0,10), clean-fleece mass (0,23), and crimp frequency (0,18). A high positive genetic correlation between weaning mass and mass at 11 and 15 months was obtained. A similar phenotypic as well as genetic association was observed between greasy-fleece and clean-fleece mass.