This article describes a procedure for conducting crosses between different strains of Ectocarpus. Crossing gametophytes to obtain the sporophyte generation is the most technically challenging stage of this process because diploid sporophytes have to be distinguished from the haploid partheno-sporophytes that result from the parthenogenetic germination of unfused gametes. This requires careful monitoring of the progeny of the genetic cross until they have developed sufficiently to be transferred to a separate Petri dish. Genetic crosses allow several classical genetic methodologies to be applied in Ectocarpus, including allelic complementation tests, backcrosses, combination of different genetic mutations, and outcrosses to create mapping populations.