The green alga Mesostigma viride has attracted considerable recent attention as a model system useful for understanding the evolutionary origin of the Viridiplantae (all green algae plus embryophytes) or the Streptophyta (charophycean algae plus embryophytes). Mesostigma's utility for such studies depends on its phylogenetic position, which has been controversial. Phylogenetic analyses based on organellar genes have given conflicting results; some support a hypothesis that Mesostigma represents the first branch of the Viridiplantae, whereas others support an alternative hypothesis that it occupies a basal position within Streptophyta. Nuclear-encoded actin and small subunit ribosomal gene sequences also mark Mesostigma as a basal lineage in Streptophyta. We tested these two alternative hypotheses by analyses of nuclear-encoded α- and β-tubulin sequences. We obtained new α- and β-tubulin gene sequences from M. viride as well as from the charophycean Coleochaete scutata and the green flagellates Pterosperma cristatum, Nephroselmis rotunda, and Mantoniella squamata, each representing a major clade within Prasinophyceae, in order to increase the taxon richness of the databases. We employed Bayesian and maximum likelihood procedures and paid particular attention to codon position effects on the phylogenetic analysis. On balance, our results support the hypothesis that M. viride is a basal streptophyte.