ABSTRACT. – This paper establishes the appropriate quadrat sample size and the minimal sampling effort to quantify the abundance and distribution of the common sub-tidal, rocky-bottom echinoderms of the Azorean Oceanic Islands. Seven species (Asteroidea: Marthasterias glacialis and Ophidiaster ophidianus; Echinoidea: Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis; and Holothuroidea: Holothuria tubulosa and Holothuria sanctori) were identified as the commonest echinoderms at São Miguel island. Two quadrat sizes were tested: the standard 1 m2 and a second quadrat size based on maximum organism size (one order of magnitude). While a 1 m2 quadrat was used for most species, due to the relatively large size of some seastars a 9 m2 quadrat was more appropriate. Five levels of sampling effort (based on the number of quadrats) were also tested. Depending on the echinoderm species, 10, 15 or 20 quadrats were most appropriate to represent species abundance. No previous study has determined the sampling unit size for quantification or determination of the spatial structure of an echinoderm species. Clearly the sample unit size needs to be determined in relation to the size of the target species together with the aggregative behavior of the species. These results provide key information for more detailed studies of the biology of echinoderms leading to establishing protocols for their conservation and management