The petal width; sepal, pistil, and stamen lengths; and changes in pollen sizes associated with heterostyly of different morphs in populations of Linum aretioides Boiss. were investigated. Fruit sets were compared in 2 populations and it was investigated whether the Bozdag population has a heteromorphic incompatibility system. It was observed that the stamen lengths of thrum morphs of Bozdag were shorter than those of thrum morphs of Babadag. Pistil lengths of both morphs of Bozdag were longer than those of morphs of Babadag. Pollen shapes of both morphs were prolate-spheroidal and subprolate, respectively, in the Bozdag population and subprolate in the Babadag population. It was determined that pollen exine in the short-styled and long-styled flowers was dimorphic. The pollen bore a marginal ring of 4 to 8 spinules in the thrum morphs and of 3 to 7 spinules in the pin morphs. It was found that the highest rates of fruit set occurred during cross-hand pollination from pin morphs to thrum morphs and from thrum morphs to pin morphs (77.77% and 71.42%, respectively). It was shown that in L. aretioides self-pollinations and pollinations between plants of the same morph are incompatible.