This study identified the most suitable fluorochrome to mark prodissoeonch of larval Siphonaria australis, a common pulmonate gastropod commonly found on rocky shores throughout New Zealand. The pulmonate habit of laying attached larval egg masses makes them an excellent candidate for studies of larval connectivity using chemical staining methods. Larval S. australis egg masses were immersed in 200 mg/L of either alizarin red S (ARS), calcein, or Oxytetracycline (OTC) for 24 h before viewing under ultraviolet light to allow quantification of mark brightness using image analysis software. Counts of pre- and postimmersion mortality were also made to identify any fluorochrome toxicity to S. australis larvae. Calcein was found to be the most suitable fluorochrome to use when fluorescently marking larval S. australis, because average brightness was significantly greater than either ARS or OTC, with negligible mortality levels recorded for calcein. The results show that fluorochromes can penetrate the gelatinous egg matrix and the larval capsules, thus providing an opportunity for staining large numbers of these species for alongshore connectivity studies.