Lythrum junceum was found to be tristylic in Israel. The three floral forms differ in style length, stamen length, pollen size, presence or absence of starch in the pollen and in stigma size. In the majority of natural populations examined, the three forms were represented in roughly equivalent numbers. The results of an artificial pollination programme demonstrate that the trimorphism is accompanied by physiological incompatibility, typical to tristylic plants. Self-pollinations were sterile. Intra-form and inter-form pollinations between anthers and stigmas at different levels were also sterile. The inhibition of pollen tubes occurs in the style. Inter-form pollinations between anthers and stigmas at equivalent level resulted in seed production, the most fertile form being the mid-styled one; these differences were lost in open-pollinated plots. Tristyly in L. salicaria with 2n= 30 is recorded. A comparison is made between tristyly in L. junceum and L. salicaria and it is suggested that tristyly in the latter has evolved later than in the former.