Urease activities were found in 2 of 3 species of trypanorhynch cestodes examined; no activity was detected in any of 7 species of tetra-phyllid cestodes. Homogenate preparations of Lacistorhynchus tenuis from the smooth hound (Mustelus canis) exhibited activities in excess of 3000 micromoles urea hydrolyzed per gram wet tissue per hour; those of Pterobothrium lintoni from the northern sting ray (Dasyatis centrura) showed activities exceeding 30,000 micromoles per gram per hour. The Lacistorhynchus enzyme showed pH optima at about pH 7.0 in both Sorensen''s phosphate buffer and Tris-maleate buffer, no optimum being observed at pH 8.0 in Tris-sulfate as has been reported for jackbean urease; saturation occurred in 250 m[image] - 350 m[image] urea; with 300 m[image] urea homogenate activities followed zero-order kinetics for at least 4 minutes when the final tissue concentration is about 2% by weight; activity was linear with protein concentration over the range examined (0.068-1.09 mgm protein). Activities of intact Lacisto-rhynchus measured about one-tenth those of its homogenate preparations; the intact worms retained full activity for about 110 hours at O[degree]C, but activities of homogenates repidly declined to about 7% of their initial activities after 24 hours at either O[degree]C or 20[degree]C. Production of ammonia and 14CO2 from [14c] urea was demonstrated using intact worms. The tapeworm urease activities were compared with those reported from other invertebrates and with that of jackbean urease.