<p>This work represents the most complete approximation to date of the taphonomy, biostratigraphy and bioevents recorded in the foraminifera of the Upper Pliensbachian-Middle Toarcian in the western sector of the Basque-Cantabrian Basin (Spain). The foraminifera are preserved as skeletal remains or internal moulds, consistent with resedimented elements with minimal transport, autochthonous or para-autochthonous. The assemblages are typical of the Atlantic Boreal Domain and inhabited a marine platform of normal salinity. The Lagenina Suborder is the most abundant and diverse, and the Miliolina, Robertinina, Spirillinina and Textulariina suborders are accessory in most of the samples. A foraminiferal biozonation calibrated against the standard ammonite biozonation is proposed. The Upper Pliensbachian to Middle Toarcian interval is divided into two biozones (Prima and Chicheryi) and five subzones (Sublaevis, Obonensis, Aragonensis, Clathrata and Iberica). The biotic crisis recorded globally in the Tenuicostatum Chronozone has also been detected in this basin. This crisis produced the stepwise extinction of typical Lower Jurassic species but without affecting generic and supra-generic taxonomic categories. Several Pliensbachian-Toarcian transition and Lower Toarcian characteristic species, such as <i>Eoguttulina palomerensis</i>, <i>Planularia obonensis</i> and <i>Saracenella aragonensis</i>, previously identified in different Spanish and European basins, have also been recorded in the Basque-Cantabrian Basin. After the extinction, a bioevent of diversification takes place in the Serpentinum Chronozone with the appearance of the genus <i>Citharina</i> and other characteristic taxa of the Toarcian and Middle Jurassic. The composition of the assemblages and the recorded bioevents are comparable to those of other areas in the Iberian Peninsula and northwestern Europe.</p>