Eight taxa of the genus Tisbe, with up to 28 laboratory populations, were analysed at four polymorphic loci (phosphoglucose isomerase, phosphoglucomutase-1, arginine phosphokinase-1 and -2), by means of vertical polyacrylamide eel electrophoresis. - Despite high intraspecific variability, and partly interspecific identity of alleles (electromorphs), the majority of the studied taxa could clearly be discriminated by at least one diagnostic locus (Nei'S genetic identity I=0) in paired comparisons. Relatively greatest discriminative power was demonstrated in arginine phosphokinase (APK-1), with 19 species pairs being differentiated, accepting probability ≥ 99 % correct diagnosis. Nevertheless, in the highly variable phosphoglucomutase (PGM-1) 14 pairs proved to be distinguishable. Including level of probability ≥99%, only one species pair (T. holothuriae Venice - T. battagliai) could not be differentiated by any of the loci examined; hence, they must be considered as the most allied of all taxa studied. - Means of genetic distance indicated different levels of divergence in the taxa, and might reflect stages in speciation process. Compared to means of genetic identity within local populations (I=0.86-1.00), the similarity between T holothuriae Venice and T. battagliai (I = 0.78) was found to be characteristic rather of the sub- or semispecies level according to AYALA (1975). Genetic affinities within three pairs T. holothuriae Venice - T. holothuriae Helgoland (I = 0.53), T. cucumariae - T. gracilis (I = 0.56), and T. bulbisetosa - T. furcata (I = 0.56), fell into the range generally covered by sibling species. The majority of the remaining taxa was well separated at the (non-sibling) species level indicated by genetic identities I ≤ 0.34. - According to the phenetic tree based on identity coefficients (UPGMA), three sets of taxa could be differentiated with their respective members probably exhibiting closer genetic interrelations: (a) T. holothuriae (Helgoland) - T. holothuriae (Venice) -T. battagliai, (b) T. bulbisetosa - T. furcata - T. clodiensis, (c) T. gracilis - T. cucumariae.