<p>The foraminiferal assemblages recorded in the materials from the Toarcian-Aalenian transition in La Fuente del Molino section (Guadalajara, Castilian Branch, Iberian Range) are described for the first time, in order to characterize the paleoenvironmental conditions in which they developed along the Mactra Subzone (Aalensis Biozone, upper Toarcian)-Bifidatum Subzone (Opalinum Biozone, lower Aalenian) interval in this area. The taxonomic composition was assessed, and morphogroup and life mode analyses were applied using the TROX model. The assemblages are dominated by several genera of the Suborder Lagenina, being <i>Lenticulina</i> the most abundant among them. The morphogroup and microhabitat analyses show the dominance of calcareous over agglutinated forms, with most specimens corresponding to the calcareous morphogroups K (<i>Lenticulina</i>) and J2 (<i>Astacolus</i>) and the agglutinated form C2 (<i>Ammobaculites</i>). The analysis of life modes shows the dominance of infaunal over epifaunal foraminifera throughout the studied interval. Morphogroup K, considered as potentially deep infaunal, has been analyzed individually and compared with the rest of the morphogroups, obtaining that the main microhabitat occupied by <i>Lenticulina</i> during most of the studied interval was probably the shallow infaunal. The TROX Model shows that the paleoenvironmental conditions in this section across the upper Toarcian-lower Aalenian transition were mesotrophic, dominated by shallow infaunal forms and a scarce representation of deep infaunal forms. The variations in the abundance, species richness, and diversity indices suggest minor environmental fluctuations across the Mactra-Aalensis boundary and the Toarcian-Aalenian transition, though none severe enough to hinder foraminiferal development.</p>