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We determined DNA sequences of SSU rRNA genes in twenty-nine polyneopteran insect species and aligned these with homologues from eight other insects. In a phylogenetic analysis we recovered the classic divisions of Palaeoptera and Neoptera, with the latter divided into monophyletic Paraneoptera and Polyneoptera. The polyneopterans divided into three lineages: one includes the Grylloblattodea, Dermaptera and Plecoptera, the second contains the Blattodea, and the third (Orthopteroidea sensu Hennig) contains the Embiidina, Phasmida, and Orthoptera, in that order. The monophyly of the Orthoptera is supported by the analyses, as is the separation between taxa from its suborders Caelifera and Ensifera. The Caelifera are not always supported as a monophyletic group; the basal Tridactyloidea are separated from the rest of the Caelifera in some analyses. Inside of Tridactyloidea, the Acridoidea, Pamphagoidea, Pneumoroidea and Trigonopterygoidea are always recovered as a monophyletic group. We also examined the basal orthopteran relationships, with the specific aim of assessing the antiquity of the Ensifera. Character state reconstructions indicated that the ancestral ensiferan sequence is very similar to the ancestral orthopteran sequence. However, likelihood ratio tests rejected the null hypothesis of a molecular clock and we conclude that a change in substitution rate has occurred within the Orthoptera and several of the other polyneopteran orders. Similar observations have been made in holometabolous insects, suggesting that variation in substitution rate is a general feature of insect nuclear rRNA evolution.