Streptograptus is a common, diverse and stratigraphically important Telychian graptolite genus. It is characterised by the presence of a centrally‐placed nozzle in the metatheca. Five species groups of Streptograptus are recognised, based upon the presence or absence of thecal characters, the most important of which are (all terms are new): cupulae, which are protuberances at the base of the theca; the platform, a concave depression with a central longitudinal convexity in which the metatheca sits; the distal lobes, extensions of the dorsal metathecal wall; and the apron, secondary fusellar growth over the original metatheca. Pseudostreptograptus thecae lack the central longitudinal convexity to the platform and bear lateral spines/spatulate processes. Isolated material of 12 species, mostly from Osmundsberget, Sweden, is described. Cupulae, the platform and ventral rhabdosome curvature appeared at the beginning of the Telychian, the apron at the beginning of the turriculatus Zone. In the subsequent crispus and sar‐torius zones some new character combinations arose (platforms in proximal thecae only of S. crispus, a species that lacks cupulae), whilst others reached their greatest recorded level of development (central longitudinal convexity and marginal flanges to platform in S. loydelli, apron in S. sartorius).