Abstract A collection of 141 bone and antler tools and debitage pieces recovered from the River Užava at the village of Sise constitutes the largest Mesolithic osseous assemblage in western Latvia. Radiocarbon dating of 12 pieces suggests that most of this collection dates from the 6^th millennium calBC. We present a general analysis, highlighting typical and unique tool forms, ornamented and sculpted pieces, and assess the corpus in a wider geographical context. Predominant in this rich and diverse collection are heavy duty antler tools: various forms of adzes, axes and hammers as well as sleeves, made either from shed antler or antler of hunted animals. They include two pieces classifiable as T-axes. Spear- and arrowheads as well as daggers are also present, along with chisels, wedges, awls and other tools. Artistic representations include five sculpted and engraved objects. The heavy duty red deer antler tools have parallels in the region south of the Baltic Sea, whereas the bone projectile forms are familiar from Kunda and Narva Culture sites of the East Baltic; the closest similarity is with osseous assemblages from coastal western Lithuania.