Bacterial diseases of sea-urchins have been reported to cause mass mortality in most oceans, but also in aquaculture. These infections cause lesions on the sea urchin adults’ body, resulting in spines, podia and pedicellaria loss. Typical lesions have a green to black central area where the calcareous skeleton is exposed to the external medium and a peripheral belt of necrotic epidermal tissues surrounding it, in many cases resulting in the death of the individuals (Becker et al., 2008; Jangoux, 1990). Pathogens that affect sea-urchin include marine bacteria, most notably Gammaproteobacteria and Vibrionaceae, as well as protists and viruses (Smith et al., 2022). With the sea urchins´ aquaculture production, there is an increasing need to understand the mechanisms and agents responsible for their pathologies. However, this knowledge is still quite scarce, being practically non-existent for the larval rearing stage. In the present work the impact of different microalgae diets on the survival and development of Paracentrotus lividus larvae was tested, while the bacteriological load of the larval culture medium was analyzed. The diets used were: D1- Rhodomonas spp., D2- Skeletonema costatum, D3- Mix of the two microalgae. At days 2, 8, 12 and 15 days after hatching (DAH), biometric sampling of the larvae was performed, and mortality was estimated. On the last day of the trial (15DAH), water samples were taken from the larval culture medium for bacterial analysis. Microbiological quality of the water was analyzed using TSA (non-selective medium) and TCBS (Vibrio-selective medium). The inoculated medium was then incubated at 23°C for 48 hr. Isolated bacteria were subjected to taxonomical analysis according to Bergey and Holt (1994), and were identified by using API20 NE system. Larvae fed the D2 and D3 diets had higher rates of survival at 15 DAH (55.8% and 39.9% respectively) compared to D1 diet (5.5%). Larvae fed with S. costatum showed a lower development than other diets. According to the microbiological analysis, water samples were taken from D1 water and from the Rhodomonas spp. culture medium both contained a significant number of bacteria from the genus Vibrionaceae. The genus Vibrionaceae and total bacteria levels were low in the microalgae S. costatum culture but were relatively high in the replicated D2 analysis. It was later discovered that Vibro alginolyticus (96.51% identification score) and Vibrio pectencida (65.70% identification score) were the two species identified. Through this work the impact that bacterial contamination from feed (microalgae) can have on the survival of P. lividus larvae was verified, also confirming the pathogenicity of the genus Vibrionaceae in sea urchins. In larval rearing of P. lividus the importance of a close control of microbial quality during microalgae production and feeding should thus be considered. Such control should include efficient disinfection and periodic analysis of both the water used and the microalgae inoculum, to avoid pathogenic bacteria contamination.