same approach as in our study of L1544. Repeating the L1544 study carried out by Caselli et al. ([@cas02a], [@cas02b]) is important because it allows us to check to what extent L1544 is an exceptional case. In order to do this we need another source which has the same general characteristics as L1544. The source selection was made using some preliminary results we obtained in a survey carried out at the IRAM-30m telescope. L1521F stood out as being the only core in Taurus, besides L1544, with strong (2-1) emission compared to (1-0). This suggests enhanced deuterium fractionation implying an advanced evolutionary state (Caselli et al. [@cas02b]). Previous observations of this object have been carried out by Mizuno et al. ([@moh94]), Onishi et al. ([@omk96]), Codella et al. ([@cod97]), and Lee et al. ([@lom99]). Onishi et al. ([@omf99]) also studied L1521F (which they call MC 27), and found a high central density, suggesting that this is the most evolved starless condensation in Taurus. L1521F was also noted by Lee et al. ([@lmt99]) as a strong infall candidate, in their survey of CS and  lines in starless cores, although later mapping of the two tracers has shown extended “red” asymmetry in the 