This paper provides articulatory and acoustic data on voiceless implosive stops in Seereer-Siin, an Atlantic language of the Niger-Congo family spoken in Senegal. Seereer is characterized by pairs of voiced and voiceless implosive stops in three places of articulation. These pairs are phonemically contrastive in lexical items. Oral air pressure measurements from Seereer stops uphold Clements & Osu's (2002) proposal that implosives and other non-explosive stops are characterized by the absence of positive oral air pressure rather than the presence of negative oral air pressure during occlusion. Acoustic data show that voiceless implosives are characterized by a short period of silence ranging from approximately twenty to fifty milliseconds before the onset of prevoicing prior to release. These findings replicate to a certain extent those of Faye & Dijkstra (1997).