The ribosomal internal transcribed spacer ITS2 is probably the most popular nuclear DNA marker used to examine relationships among and within species in animals and plants. ITS2 sequences have also begun to be used as DNA barcodes. ITS2, however, has rarely been used in studies of spiders. Here, I examine the potential utility of this marker for spider phylogenetics based on preliminary data for Anelosimus spiders and a brief summary of prior work. The secondary structure of ITS2 facilitated alignment of highly divergent sequences and indicated that secondary structure morphology might be phylogenetically informative in itself. Phylogenetic analysis of Anelosimus species was congruent with a prior study based on a combination of six mitochondrial and nuclear loci plus morphology regarding the deeper clades within the genus. However, ITS2 had insufficient variation to resolve relationships within species and among closely related species. Previous studies have also discovered relatively little within-species variation in ITS2. In sum, ITS2 is an easily amplified and sequenced marker that is underutilized in spider phylogenetics; however, it has limited uses at the lowest taxonomic levels and is not likely to be a universally useful DNA barcode marker.