Mammoth Uncensored Word List:
20 words ending with st, 19 letters long

anticonservationist antifluoridationist chemicophysiologist chemostratigraphist cyclostratigraphist electrometallurgist electrophysiologist lithostratigraphist micropaleontologist neuropharmacologist palaeoclimatologist paleoanthropologist paleotempestologist phytopaleontologist representationalist syncytiotrophoblast transformationalist ultrafundamentalist ultrasegregationist ultratraditionalist