Earlier work identified two novel strains of probiotic bacteria, Alteromonas macleodii 0444 and Neptunomonas sp. 0536, for Greenshell™ mussel (Perna canaliculus) larvae. Herein, we investigated whether the combination of the two probiotics in a multi-strain mix provided a) enhanced larval production during routine rearing and b) improved larval protection during two separate pathogen-challenge tests (Vibrio sp. DO1 and Vibrio splendidus). The response of larvae to multi-strain or single-strain probiotic administration was compared to that of larvae without probiotics. Two concentrations of each probiotic were tested (10⁷ and 10⁸ CFU ml⁻¹). Addition of 10⁸ CFU ml⁻¹ multi-strain mix in the routine rearing of larvae yielded smaller-sized larvae and a lower feeding rate when compared with the 10⁷ CFU ml⁻¹ and control groups. During the challenge test against Vibrio sp. DO1 and V. splendidus, protection by single-strain probiotic administration was observed at both levels of probiotic administration (10⁷ and 10⁸ CFU ml⁻¹), with no apparent added protection from multi-strain probiotics. Although 10⁸ CFU ml⁻¹ levels provided protection against pathogen attack, they were also potentially detrimental to normal larval rearing when administered in combination and, as such, where administration of A. macleodii 0444 and Neptunomonas sp. 0536 would be applied together, a multi-strain mix of probiotics at 10⁷ CFU ml⁻¹, is recommended as the best concentration of each probiotic.