Tuberization in potato is known to be under complex biochemical control involving hormones. A number of studies have provided evidence for a critical role of GA in tuberization. There is also evidence that GA in plants can be modulated by a Ca/calmodulin pathway. The purpose of the present study was to determine the influence of supplemental Ca fertilization on tuber size and tuber number. Plantlets of Solanum tuberosum `Russet Burbank' raised in tissue culture were planted in 20-L pots filled with sandy loam field soil with the pH of 6.9 and exchangeable soil Ca level of 350 ppm. All treatments received the same total amount of N (equivalent to the rate of 280 kg·ha-1). Four treatments were evaluated: nonsplit N (from ammonium nitrate), split N (from ammonium nitrate), split N+Ca (from calcium nitrate), split N+Ca (50% N from urea, 50% N from ammonium nitrate and Ca from calcium chloride). The total Ca was applied at the rate equivalent to 168 kg·ha-1 on a split schedule (equally split at four, six, eight and ten weeks after planting). Four months after planting tubers were harvested and evaluated. As expected tuber tissue Ca was increased by Ca application from 144 to 245 μg·g-1. In general, the two Ca treatments had significantly lower tuber number per plant as compared to the nonsplit and split N treatments. A plot of mean tuber Ca and tuber number for individual plants showed a significant negative relationship. Both Ca treatments produced tubers with higher mean tuber weight compared to nonsplit N. This increase in tuber size with Ca application was not apparent when compared with split N treatment. These results show that Ca application to soil can decrease tuber number suggesting that soil Ca may influence tuberization in potato.