188 male and 188 female one-day-old ISA Vedette broiler chicks were fed ad libitum with standard broiler diets, DKA-starter (to the age of 3 weeks), DKA-grower (from 4 to 6 weeks) and DKA-finisher (from 7 to 12 weeks). From week 3 on, 10 cockerels and 10 hens were slaughtered at weekly intervals and the carcasses were dissected. The optimum fattening period for ISA Vedette broiler chickens ranges between 6 and 8 weeks. When the fattening period was extended towards the upper limit, the following positive effects were observed: increased dressing percentage, lower costs of purchasing chicks per unit of product and higher European Production Index. On the other hand, some drawbacks occurred, such as decreased feed conversion, higher costs of feed and higher fat content in the carcass. The unfavourable effects were more intense in females than males as the birds grew older. Consequently, the fattening period should be longer for cockerels and shorter for hens.