Biodiversity and taxonomy of punctoid micromollusks from southern Brazil, with description of a new species of Radiodiscus from the «Mata Atlántica» (Atlantic Rain Forest) (Mollusca, Gastropoda, Pulmonata). Species in Charopidae and Punctidae from the «Centro de Pesquisas e Conservação da Natureza Pró-Mata (CPCN Pró-Mata)» (Brazil, Rio Grande do Sul) and Thiele's collection are studied. The native species Rotadiscus amancaezensis (Hidalgo, 1869), Radiodiscus cuprinus Fonseca & Thomé, 2000, R. compactus (Suter, 1900) comb. nov., R. promatensis Miquel, Ramírez & Thomé, 2004, Lilloiconcha superba (Thiele, 1927, L. zulmae Miquel, Ramírez & Thomé, 2004 comb. nov., L. gordurasensis (Thiele, 1927, and Radioconus pleurophorus (Moricand, 1846) comb. nov., and the probably exotic, Sinployea? goeldii (Thiele, 1927) were present in the charopid samples from the CPCN Pró-Mata; punctids were represented by Paralaoma servilis (Shuttleworth, 1852) and Paralaoma sp. A new species of Charopidae is described: Radiodiscus sanchicoensis sp. nov. It is a tiny shell with 75 to 60 axial ribs in the last whorl. The protoconch microsculpture of Lilloiconcha, Zilchogyra and Radioconus, with generic value, is studied, as well as their internal calcareous teeth. Several species and a genus (Lilloiconcha = Trochogyra) are places into synonymy. The actual presence of some Patagonian species in southern Brazil is discussed; the only charopid reported to be present in Patagonia and Brazil is R. amancaezensis. R. sanchicoensis is the dominant species in the CPCN Pró-Mata, followed by R. cuprinus.