When generating kelp hybrid crosses in vitro, small blades are frequently seen in single-sex cultures of microscopic gametophytes as well as in putative hybrid crosses. We demonstrate a polymerase chain reaction (PCR)-based method that quickly determines the presence/absence of possible parental genomes in the observed blades. ITS1 nrDNA fragments that varied in length in a species-specific way were amplified. Amplification products from possible hybrid blades, apogamic blades and parental gametophytes were compared on agarose gels. Using these methods, we were able to identify a number of apogamic blades (i.e. formed directly from gametophytic cells) as well as a true hybrid sporophyte blade in crosses between Alaria marginata (Alariaceae) and Lessoniopsis littoralis (Lessoniaceae), members of two separate families of the Laminariales. To our knowledge this is the first genotypic confirmation of a hybrid kelp.