Here we show, by using full atomic molecular dynamics simulations, that electrostatic correlations typical of multivalent ions can reverse the selectivity of a biological nanochannel. We analyze the ionic transport and partition across the channel in the case of monovalent, divalent and a mixture of ions. Our results provide a physical mechanism for a new, experimentally observed phenomenon, namely the inversion of the selectivity of a bacterial porin (the E. Coli OmpF) in presence of multivalent cations. Also, the differences and similarities between this phenomenon and other nano and micro-scale electrokinetic effects are briefly discussed. © 2010 The Royal Society of Chemistry.