Taxocoenosis of thecamoebae from water bodies in the middle Parana River islands I. Thecamoebae from the floating plants in the ox-bow "Don Felipe". 56 taxinomic entities summing up 43 species of Thecamoebae are described and illustrated. The list includes a change in denomination (Difflugia (Pseudocucurbitella) multidentata nov. comb. pro Diff. lobostoma var. multidentata G-L y Th.), six new varieties and forms (Centropyxis hirsuta fma. irregularis f. nov., Difflugia ventricosa fma. recticaulis f. nov., Protocucurbitella coroniformis fma. acuminata f. nov., Protocucurbitella coroniformis var. tuberculata nov. var., Cucurbitella obturata var. micropyla nov. var., Lesquereusia modesta var. mucronata nov. var.) and four new species (Centropyxis plagiostoma nov. sp., Difflugia pleustonica nov. sp., Cucurbitella neotropicalis nov. sp., Cucurbitella microstoma nov. sp.).