The effect of environment on the relationship between grain carbon isotope discrimination (Δ) and yield was studied for durum wheat (Triticum turgidum L. var. durum) under Mediterranean conditions. A group of 25 genotypes was grown under contrasting water regimes in two regions of Spain during three years. The first objective was to determine the environmental factors responsible for the strong positive relationship previously observed between Δ and yield across trials. Environmental factors tested were total water input (W_i), mean temperature, accumulated reference evapotranspiration (ET₀), and the ratio W_i/ET₀ during different periods of the crop cycle. Water input during grain filling was the variable most strongly correlated with grain Δ and yield across all the trials, as well as across the subset of trials in northeastern Spain. In southeastern Spain, the most drought prone of the two regions, W_i from sowing to heading explained the most variation in grain Δ and yield. The second objective was to study the effect of environment on the relationship between Δ and yield across genotypes. No significant correlation was found for trials with a mean yield up to about 2000 kg ha⁻¹, but the strength of the relationship increased sharply and attained significance in trials yielding 2500 kg ha⁻¹ When yield above 2500 kg ha⁻¹ the correlation between Δ and yield remained relatively steady and positive, with an r value around 0.5. It is concluded that breeding to raise durum wheat yield in Mediterranean conditions could take advantage of selecting for higher Δ only in relatively wet years or under supplementary irrigation.