A three-dimensional network of structural filaments was visible with common electron microscopes in the cytoplasm of Euglena gracilis green cells extracted with buffers containing the nonionic detergent Triton X-100. A similar filamentous web was detected at the periphery of critical point dried cells cleaved on grids by means of an adhesive tape. SDS-polyacrylamide gel electrophoresis of the detergent-resistent cytoskeleton showed that actin or actin-like proteins of molecular weight in the range of 43–45 K are not among the components having a structural role inEuglena. The significance of these findings was discussed in relation to the capability of the alga to change the cell shape.