Ride　comfort　in　vehicles　can　be　affected　by　seat　properties.　While　previous　studies　focused　on　the　vertical　whole-body　vibration,　this　study　was　designed　to　understand　the　influence　of　the　foam　thickness　at　the　seat　pan　and　at　the　backrest　on　seat　transmissibilities　with　fore-and-aft　vibration.　Twelve　subjects　sitting　with　or　without　a　vertical　backrest　were　exposed　to　random　fore-and-aft　vibration　between　1　and　15 Hz　with　the　magnitude　of　0.5 ms-2　r.m.s..　It　was　found　that　there　was　no　significant　difference　in　the　primary　resonance　frequencies　in　the　fore-and-aft　in-line　and　vertical　cross-axis　transmissibilities　to　the　seat　pan　and　to　the　backrest.　The　resonance　frequency　in　the　fore-and-aft　in-line　transmissibilities　to　the　seat　pan　and　the　backrest　decreased　with　increasing　thickness　of　foam　at　the　seat　pan　and　the　backrest.　Altering　the　thickness　of　foam　at　the　seat　pan　was　more　effective　than　changing　that　at　the　backrest.