Marine protected areas (MPAs) are important tools to achieve ecological, cultural and socioeconomic goals. MPAs ecological management eff ec veness (EME) is the degree to which MPAs reach their ecological goals. The signifi cant variability of EME among MPAs has been extensively studied and partly explained by several MPA features linked to design, management and implementa on (e.g. surface area, enforcement, age of protec on). We inves gated EME variability by employing, for the fi rst  me, organiza on science, the discipline that studies organiza ons. Using an exploratory mul ple case study approach, we applied OS principles to eleven Mediterranean MPAs in order to characterize several organiza onal features. Among those eleven, eight MPAs were taken into account to explore the rela onships between EME and MPA features, such as: 1) organiza onal size (i.e. the ra o between the number of full- me employees and the total MPA surface area), 2) management performance (i.e. the 

level of eff ort exerted to enhance and sustain the MPA management, including enforcement), 3) total surface area, and 4) MPA age. The log- response ra os of fi sh biomass in protected vs. unprotected (control) areas was used as a proxy of EME. Results show that a number of organiza onal features are highly variable among the MPAs we studied. Management performance, organiza onal size and, to a lesser extent, MPA age were posi vely correlated with the log-response ra o of fi sh biomass, whereas total surface area did not display a signifi cant role. Based on our fi ndings, two relevant outcomes emerge from this exploratory study: i) the management performance of the MPAs considered needs extensive improvements; ii) the employment of organiza on science in the management eff ec veness assessment made possible the elabora on of a framework (see image) that can assist MPA managers to reach goals more effectively, with a more effi  cient use of available resources.