This study evaluated the effect of rate and timing of N fertilizer application on tuber yield, tuber size distribution and tuber specific gravity of potato (Solanum tuberosum L.) in two sites in a commercial field in 1999–2001. One trial was established at each of the two sites chosen to represent two management zones (MZ) previously delineated by soil electrical conductivity, and differing in soil water availability, as controlled by depth to a clayey substratum. Each trial had 21 treatments including five rates of ammonium nitrate (0–200 kg N ha-1 in 1999; 0–240 kg N ha-1 in 2000 and 2001), each applied according to five application timings (100, 75, 50, 25 or 0% of N applied at planting with the remainder applied at hilling). Increasing N rate increased total and marketable tuber yields in both sites. The N rates required to optimize economic return ranged from 167 to 239 kg ha-1 among years and between sites. Nitrogen fertilizer applied all at planting or all at hilling decreased tuber yields in both sites compared with split N application. The percentage of N rate applied at planting to achieve the maximum marketable tuber yield varied among years from 34 to 61% and allowed a yield increase of 2 to 20%. The proportion of large tubers was higher in the site with a greater depth to the clayey substratum (DMZ site) than in the site with a shallower depth to the clayey substratum (SMZ site). Tuber yield and tuber N uptake were higher in the SMZ site than in the DMZ site when no fertilizer N was applied, and tuber yield was more responsive to fertilizer N rate in the DMZ site than in the SMZ site in one year; however, these differences are not sufficient to justify different fertilizer N management for the two sites. The two sites frequently differed in terms of tuber yield, tuber size and specific gravity, which are important parameters in determining tuber processing quality but the optimal N rate and N timing were similar. These differences may be sufficiently large to justify different potato management practices (e.g., nutrient management, seedpiece spacing) to optimize potato production for the chip processing market. Key words: Solanum tuberosum, marketable yield, tuber size distribution, specific gravity, N economic optimum