The neutron drip line up to fluorine has been investigated by the projectile fragmentation of a 94.1A MeV 4 0 Ar beam at the fragment separator RIPS at RIKEN. A new neutron-rich isotope, 3 1 F, has been observed for the first time while clear evidence for the particle instability of 2 4 , 2 5 N, 2 7 , 2 8 O and 3 0 F has been obtained. The sudden change in stability from oxygen to fluorine may demonstrate the onset of deformation for the neutron-rich fluorine isotopes.