The effects of five diets, with 12–18% crude protein (CP) and 6.6–10.8 g lysine per MJ digestible energy (DE), on live performance, carcass and meat quality criteria were recorded for 90 pigs in a split-plot design involving male castrates and gilts and three replicates, with three pigs per pen. The five diets, differing in the ratio of barley:soybean meal, were fed as pellets, ad libitum from 25 to 98 kg liveweight. A balanced incomplete block design was used with four additional pens to assess the palatability of the diets offered in pairs sequentially over 10 wk. As expected, castrates had a higher intake (2817 vs. 2580 g d−1) and daily gain (888 vs. 800 g d−1) than gilts and were relatively fatter except when fed the lowest CP diet. While an increase in dietary CP, or lysine:energy ratio, led to significant improvements in growth rate, gain:feed and lean content, there was a concomitant reduction in marbling and sensory appeal. The palatability study suggested that both castrates and gilts tended to favour the intermediate diets (8.8 and 10.1 g lysine per MJ DE) over the others, with a preference for higher CP diets in the early growth period. Estimates of the lysine:DE ratio for maximum live performance indicated that gilts should receive > 10.8 in the pre- − 50 kg period and approximately 10.1 g lysine per MJ DE thereafter. For castrates, the respective ratios were 10.8 and 8.8 g per MJ DE. Split-sex feeding, with gilts receiving about 2% higher dietary protein levels than castrates, was also favoured by the estimates of economic benefits for the producer. Key words: Pigs, lysine, digestible energy, carcass, pork quality