Natural populations of hybrids and populations of parental species of the Carex flava aggregate, growing in seven localities in Poland, were studied. Statistical methods were used to analyse 30 morphological characters measured from 399 dried specimens. The results allowed us to recognize the following hybrids : C. × alsatica [C. flava × C. demissa], C. × ruedtii [C. flava × C. lepidocarpa], C. × schatzii [C. lepidocarpa × C. viridula] and C. demissa × C. viridula. We found that (i) the utricle and beak lengths in the hybrid are usually similar to those of the parents that have longer utricles and beaks, and that (ii) introgression typically occurs towards the parental taxon dominant in an area and that with an earlier flowering period. The morphological characters most useful in distinguishing between hybrids and their parental species in the C. flava aggregate are the size of the utricle and the beak as well as characters transgressive in the hybrids, associated with inflorescence length, location of female spikes, and length of male utricles and their peduncles.