Recently, two non-native caprellid amphipods, Caprella scaura and Paracaprella pusilla, were found for the first time in southern Spain. In order to assess their establishment success, monitoring of the caprellid fauna associated with the marina's fouling community was carried out from summer 2010 to spring 2011 in Cádiz marina, southern Spain. Three caprellid species were found, the non-native C. scaura and P. pusilla, and the native Caprella equilibra. Caprella scaura was the dominant species with ovigerous females and juveniles throughout the whole study period, reflecting both that it reproduces all year around and that it is able to support the differences measured in water temperature, salinity and turbidity among seasons. It showed a high ability to colonize different fouling substrates, although it was significantly more abundant in the introduced bryozoan Bugula neritina. The tropical caprellid P. pusilla was found in lower abundance than C. scaura as it only was present in summer, associated exclusively with the native hydroid Eudendrium racemosum. The native C. equilibra, very common in other nearby harbours where alien caprellids were absent, was scarcely represented in the study area. The results suggest first that P. pusilla is an introduced species with less invasiveness than C. scaura, and second, that fouling bryozoans, especially B. neritina, are suitable habitat for the establishment success of C. scaura, harbouring high densities of this invasive species, while fouling hydroids, particularly in the case of the native hydroid E. racemosum, are more suitable for the establishment success of P. pusilla.