Early ontogenetic stages of some Ammonitina (mostly Perisphinctidae) are described from the Oxfordian Jagua Formation of western Cuba. Fourty six percent of the sample are specimens attaining the nepionic swelling with more
than four septa developed. Juvenile jaw apparatus found for the first time in specimens with only 2,25 whorls, displays primitive elements corresponding to conchorhynch and rhyncholit of nautiloids, litoceratids and phylloceratids. The
fauna and sediment features evidence a low water energy and low oxigenated conditions at the bottom during the sedimentation. The juvenile ammonites occur together with numerous adults which are represented by corresponding micro- and macroconchs: it indicates for the proximity of a breeding place.

Key words: ammonites, juvenile shells, juvenile jaw apparatus, paleoecology, Jagua Fm., Oxfordian, Cuba.