Historical published works on the fossil fauna of the Llanberis Slates Formation in Gwynedd, North Wales, concentrated on the endemic trilobite Pseudatops viola (Woodward, 1888), and placed the formation towards the top of Stage 3 (Series 2) of the Cambrian chronostratigraphy.The impression given was that the fauna is not particularly diverse and fossils are rare. However, recent collecting has produced a comprehensive fauna of sponges, hyolithids and diverse arthropods including over 250 trilobite specimens from two localities of the uppermost Green Slate horizon. Among them are examples of the endemic trilobite Pseudatops viola, sufficiently well preserved to enable a reappraisal of its status. The most abundant trilobite is a form of Strenuella (cf. strenua), but the collection includes previously unrecorded eodiscoid trilobites and specimens representing unrecorded and potentially new taxa. These additions result in the most comprehensive lower Cambrian trilobite assemblage yet described from Wales, and place it in the lower part of Stage 4, Series 2 in standard global terms.