AbstractTheMediterranean hydrozoan fauna (Siphonophora excluded) comprises 400 species; most (68%) occur in theAtlanticOcean, 20% are endemic to theMediterranean, 8% are ofIndo‐Pacific origin, and 4% are non‐classifiable. There are 69 nonindigenous (NIS) species in the basin: 44% of these are casual (recorded just one or very few times), 28% established (widely recorded in the basin), 6% invasive (establishedNISthat are able rapidly or largely to disseminate away from the area of initial introduction, having a noticeable impact on the recipient community), and 22% questionable (of doubtful taxonomic status). Entry through theSuez Canal and range expansion through theGibraltarStrait, often enhanced by ship traffic, appear to be the main processes for recent species introductions, but uncertainties remain for manyNIS. Species additions immediately result in larger local or regional species pools, but the newcomers might impact on populations of native species, altering extinction probabilities. A more reliable evaluation of the species pool can be accomplished by adding new species when they enter the taxonomic record (i.e. the records of any taxon in all types of literature), and by removing species that have not been found for a ‘reasonable’ time (e.g. several decades). Of the 400 non‐siphonophoran hydrozoan species known to occur in theMediterraneanSea, positive records in the last 10 years are available for 156 species (39%), whereas records of the remaining 244 species are older than a decade: 67 species have not been recorded for 41 years, 13 for 31–40 years, 79 for 21–30 years, and 85 for 11–20 years.