1. Urease from a sea urchin Lytechinus variegatus, was purified 300-fold, using heat precipitation, ethanol precipitation and gel filtration. 2. The pH optimum is 8.0. 3. The apparent Michaelis constant for urea is 0.13 mM at pH 8.0. 4. The inhibitory effects of seven reagents on urease were evaluated. The pattern of inhibition is similar to other invertebrate ureases. 5. L. variegatus urease is compared with that of several other invertebrates, and its possible significance in CaCO3 formation is discussed.