<title>Abstract</title>In Thailand, two species of rhizostome jellyfish, <italic>Rhopilema hispidum</italic> and <italic>Lobonemoides robustus</italic>, are commercially harvested. The cnidomes, nematocyst size and toxicities were compared between these species. <italic>Rhopilema hispidum</italic> and <italic>L. robustus</italic> each had four types of nematocysts on their oral arms. For <italic>R. hispidum</italic>, these nematocyst types included two types of isorhiza and two types of rhopaloid, while in <italic>L. robustus</italic>, there were three types of isorhiza and one type of rhopaloid. For <italic>R. hispidum</italic>, tubule lengths of the largest nematocyst type (large round isorhiza; mean ± SD = 313.8 ± 62.2 μm) were significantly longer than those of <italic>L. robustus</italic> (large ellipsoid rhopaloid; 162.1 ± 38.5 μm). Using the freshwater shrimp, <italic>Palaemon paucidens</italic>, in a bioassay, we determined that the lethal nematocyst concentrations for <italic>R. hispidum</italic> and <italic>L. robustus</italic> were 5705.3 ± 1118.1 and 3408.3 ± 1032.9 unit g⁻¹ wet weight, respectively, and that these concentrations were significantly higher in the former than in the latter.