INTRODUCTION In North America, the freshwater rhabdocoels are rather well studied, in contrast to the marine rhabdo- coel fauna. Most of the North American freshwater rhabdocoels belong to the taxon Dalyelliidae (see Graff 1911, Higley 1918, Riedel 1932, Ruebush 1937, Ruebush and Hayes 1939, Jones and Hayes 1941, Hayes 1945, Hyman 1955; for an excellent taxonomical and distrib- utional overview of all dalyelliid species known until then, see Luther 1955). Of the approximately 150 spe- cies of Dalyelliidae known worldwide, 20 species occur in the USA (Silliman 1884, Ruebush 1937, Ruebush and Hayes 1939, Hyman 1955, Luther 1955, Ax and Armonies 1990). Seventeen of them are limnic, two are marine [Halammovortex nigrifrons (Karling, 1935) Karling, 1943 and H. lewisi (Jones et Ferguson, 1948) Luther, 1955] and one (Jensenia parangulata Ax et Armonies, 1990) is a brackish water species (see Luther 1955 for an overview). The total number does A NEW SPECIES OF MICRODALYELLIA (RHABDITOPHORA: RHABDOCOELA: DALYELLIIDAE) FROM TEMPORAL FRESHWATER ROCK POOLS IN UTAH, USA ANNALES ZOOLOGICI (Warszawa), 2007, 57(3): 371-376 WIM WILLEMS 1, 2 * , TOM ARTOIS 2 , MERLIJN JOCQUÉ 3 and LUC BRENDONCK 3 1 Dept Invertebrate Zoology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden; e-mail: wim.willems@nrm.se; *Corresponding author 2 Research Group Biodiversity, Phylogeny and Population Studies, Centre for Environmental Sciences, Hasselt University, Campus Diepenbeek, Agoralaan, Building D, B-3590 Diepenbeek, Belgium 3 Laboratory of Aquatic Ecology, Catholic University of Leuven, Charles de Bériot- straat 32, B-3000 Leuven, Belgium Abstract.— A new species of Microdalyellia Gieysztor, 1938 is described from a temporal freshwater pool in Utah, USA. This new species is characterized by the detailed structure of the stylet, which is of the Microdalyellia-type and consists of a crossbeam that carries two proximal shafts, two distal branches each carrying 5-7 hollow spines, and a median process in between both these spiniferous branches. The most remarkable feature is the presence of a second connection (or crossbeam) between both shafts, giving the proximal part of the stylet the appearance of a plate with a large window. The species’ affinities within the taxon Dalyelliidae Graff, 1905 are discussed. Following the discussion, Gieysztoria rastafariae Therriault and Kolasa, 1999 is transferred to the taxon Microdalyellia, because of the presence of a Microdalyellia-type stylet, which closely resembles that of the newly described species. ｱ Key words.— taxonomy, new taxa, Platyhelminthes, 'Turbellaria', Dalytyphloplanida, Microdalyellia, Gieysztoria, rock pools, North America.