The Little Neath River Cave is situated in the Valley of the Nedd Fechan, Breconshire, S. Wales. It was discovered in 1967 and has been explored and surveyed to its current length of over 8 km by members of the University of Bristol Spelaeological Society. In dry weather the whole of the Nedd Fechan sinks into the system, mostly via Bridge Cave. The water resurges in the bed of the river 2 km to the south near Pwll Du, which is itself a flood rising. Sump 6 - the present end of L.N.R.C. is at the same altitude as Pwll Du. As with other South Wales cave systems the cave has been heavily influenced by the glacial history of the region. Successive phases of erosion, deposition and re-excavation are suggested. The development of the cave down-dip and the later change of direction of flow lines are linked to surface changes. The local flood hydrology is discussed with the conclusion that flows dangerous to spelaeologists may occur on almost any winter’s day. A description and Grade 5C survey of the cave are also presented. A checklist of smaller caves in the Little Neath Valley is also given.