During a broad study of invertebrate colonization in marginal fluvial wetlands of the Middle Paraná River in Argentina, cyst formation was observed in two of the 26 species of aquatic oligochaetes identified from the study—Dero multibranchiata Steiren, 1892 (Tubificidae, Naidinae), and Trieminentia corderoi (Harman, 1969) (Opistocystidae). All specimens of the two species in which cyst formation was observed had been collected after drying conditions ranging from 14 to 28 days. A clitellum was not observed in any of the specimens collected. Lengths of specimens ranged from 0.88 mm to 2.84 mm in Dero multibranchiata and 2.25 mm to 2.40 mm in Trieminentia corderoi. The development of protective cysts by Dero multibranchiata and Trieminentia corderoi, and perhaps many other species of aquatic oligochaetes, appears to be an adaptive strategy enabling them to survive drought in temporary wetlands, recolonize freshwater habitats upon inundation, and disperse both laterally and downstream with rising water levels in wetlands and streams.