The identification of factors determining the adaptation and nitrogen (N) utilization of spelt wheat is important for the successful introduction of the crop to a new environment. The present study was carried out to investigate the relative importance of grain-filling rate and duration of grain growth and to analyse the nitrogen utilization efficiency (NUtE) and biomass production efficiency of spelt under Mediterranean conditions. The performance of spelt was evaluated in relation to a well-adapted bread wheat cultivar. Three spelt cultivars (Ressac, Poème and Cosmos) and one bread wheat cultivar (Centauro) were grown for two growing seasons on a silty clay soil under two N levels (0 and 100 kg N/ha). Grain-filling parameters were estimated using the cubic polynomial model. This model provided good fit to the grain-filling data of spelt cultivars, with high coefficients of determination (R2) that ranged from 0·868 to 0·999. Cultivar differences were found for all grain-filling parameters studied, and these differences accounted for most of the variation observed within each particular grain-filling component in both years. Grain filling of spelt plants took place under adverse environmental conditions, mainly high temperatures, which led to a shortening of the grain-filling period. This fact was not fully compensated by the increase in the grain-filling rate, and eventually resulted in a reduction of the final spelt grain weight. Selection for early-flowering cultivars could be a successful strategy to moderate the influence of the environment on grain filling and improve the adaptation of spelt under Mediterranean conditions. The mean grain-filling rate was positively correlated with dry matter translocation, suggesting the crucial role of reserve assimilates in the vegetative tissues for the grain growth of spelt. The efficiency of N utilization to produce biomass was greater during the grain-filling period than the vegetative period. Averaged across N application rates, NUtE in spelt ranged from 20·1 to 29·5 g grain/g plant N. Cultivar differences in NUtE were observed in both years. Grain yield per unit grain N (grain DM/grain total N at maturity) contributed more to the total variation in NUtE among spelt cultivars compared with N harvest index (NHI). Spelt showed lower NUtE values, probably due to its higher grain N concentration and lower NHI compared with wheat. Low straw N concentration at maturity may be an indicator of improved NUtE in spelt, as evidenced by the negative relationship detected between the two variables. These results provide a better understanding of factors related with the adaptation and N utilization of spelt under Mediterranean conditions.