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Heterotrissocladius spiesi sp. n. is described from headwaters of two small Georgia streams, below the Fall Line in the Coastal Plain Physiologic Province, Atlantic Slope drainage. One stream is near the coast (Lower Coastal Plain) and characterized as a“blackwater” stream due to high tannin concentrations. The other stream is inland (Upper Coastal Plain), clear and not noticeably affected by tannins. The description is based on adult males, pharate male pupae and one attached larval skin. The new species is assignable to the H. marcidus species group with some exceptions. Males differ from the previous group by a reduced number of rather weak, short acrostichal setae (0-3, previously considered as typical of the subpilosus group) as well as by a very reduced wing squamal fringe compared to most members of the H. marcidus group. Males are recognizable by an elongate, thin virga, distinctive anal point and inflated/triangular gonostylus. The female is not described; the immature stages are very similar in some features to H. boltoni Sæther. At the inland stream, an adult male of the new species was found together with pharate male pupae (one with larval skin) of another, as yet undetermined species of Heterotrissocladius (marcidus group) also known from elsewhere in the state. Female adults and a pharate female pupa collected at the blackwater stream site are noted but not included in the description or type series.