The 492 GHz (~3P_1 → ~3P~0) fine-structure line of neutral atomic carbon (C Ⅰ) has been observed in the planetary nebulae (PNe) NGC 6302, IRAS 21282+5050, and NGC 7027, and in the protoplanetary nebula (PPN) AFGL 2688. The estimated C Ⅰ/CO abundance ratio is higher for a more evolved object, consistent with a trend that the C Ⅰ/CO ratio increases with the evolution of a post-asymptotic giant branch system from a PPN to a PN. Nondetections are also reported on 11 PPNe and extreme carbon stars. The upper limits placed on these objects suggest that they have C Ⅰ abundances lower than the level found in the PPN AFGL 2688, again consistent with an increasing C Ⅰ/CO ratio with evolution.
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