The growth rates of Aulacomya atra and Mytilus chilensis were assessed along a vertical gradient covering the natural occurrence of both species. Recruits, attached to branches of a submerged tree (Mytilus chilensis) and shells of adult bivalves (Aulacomya atra), were collected from the fjord as entire clusters. In the laboratory the initial length was marked by incubation in Calcein solution (200 mg/l; 6 h, 40 l aquaria with continuous aeration) and with arrows carefully scratched into the periostracum using a scalpel. The arrow tip points at the shell margin at initial length. Preliminary in situ experiments revealed the necessity to protect juvenile test bivalves from predation. Therefore five cages (40 cm x 40 cm x 20 cm) produced of wire netting (square opening width 3 cm) were installed at Station 1 (X-Huinay). Cages were placed in the intertidal (at Mean Sea Level) and the subtidal zone (5, 10, 15, 20 m). One marked cluster of each species was fixed inside the cages on 28 January 2012. Additionally, one cluster of M. chilensis, connected to the cages via a nylon line carrying a buoy, floated 0.8 m above each cage. The bivalves were recaptured on 3 March 2012 after 35 days of growing.This dataset was generated in the frame of the co-operation between the Huinay Scientific Field Station (http://www.huinay.cl) and the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (http://www.awi.de).