  We have determined fluorine abundances from the F II 4789 and F IV 4060
nebular emission lines for a sample of planetary nebulae (PNe). Our results
show that fluorine is generally overabundant in PNe, thus providing new
evidence for the synthesis of fluorine in asymptotic giant branch (AGB) stars.
[F/O] is found to be positively correlated with the C/O abundance ratio, in
agreement with the predictions of theoretical models of fluorine production in
thermally pulsing AGB stars. A large enhancement of fluorine is observed in the
Wolf-Rayet PN NGC 40, suggesting that high mass-loss rates probably favor the
survival of fluorine.
