Bintulu, an Austronesian language spoken in the district capital of that name and in several smaller communities in the basin of the Kemena river, Fourth Division, Sarawak, has two previously unrecognized implosive stop phonemes, 6 and d. Where these segments occur in lexical items known to reflect a Proto-Austronesian (PAN) reconstruction, they appear to result from an unconditioned phonemic split of PAN *b, *d, *D, or *j. A hypothesis constructed to account for a wider range of linguistic facts offers an explanation of some of these apparent irregularities. Most if not all residual discrepancies can be accommodated under an ancillary hypothesis of borrowing, independently motivated by various considerations. The case of the Bintulu implosives is of general theoretical interest, as it necessitates modifications in some recent claims regarding the diachronic sources of implosive stops.