A study was conducted in which 226 participants performed 12 tests, 6 thought to reflect verbal, quantitative, and spatial working memory (WM), and 6 of crystallized (Gc), fluid (Gf), and spatial (Gv) cognitive abilities. Confirmatory factor analyses (CFAs) were computed to test the unitary nature of the WM system. Six primary latent factors were obtained from the verbal, quantitative, and spatial tests. Three second-order latent factors were then obtained from the primary latent factors, following Mackintosh and Bennett's procedure ["Intelligence" 31 (2003) 519-531]. A very high correlation was observed among these second-order latent factors. Given this high correlation, those second-order latent factors were collapsed into a single second-order latent factor. The restricted model fitted the data as well as the base model. The result strongly supports the unitary nature of the WM system. It is proposed that this system can be conceived as a single pool of general purpose resources used to temporarily preserve a reliable mental representation of information about any task.