This data was collected at the Centre for Applied Marine Sciences as part of the REPROSEED Project. Data is presented as an Excel file in English. Data was gathered using a Leica DM E Compound Microscope. Data was analysed with Minitab statistical software. Environmental and husbandry factors play key roles in the control of byssus attachment and detachment in many bivalve species. Identifying and understanding the impact of influential factors is essential for the management of post-settlement hatchery reared juveniles in species, such as the scallop Pecten maximus. We assessed the impact of substrate type and condition, and attachment period on attachment of juvenile scallops between 1.9 and 5.9mm in shell height. Comparison of a variety of substrates showed that scallop juveniles have a preference for textured hard surfaces, with mean attachment up to 75.6±14.4% after 24 hours on riven slate in static conditions. Attachment could be significantly increased by preconditioning substrates in flow-through tanks of unfiltered seawater and by having undisturbed attachment periods longer than 1 hour. The effect of substrate type and substrate condition on retention of seed was assessed in a flume. For all substrates, retention decreased with increasing water velocity. However, of the substrates examined (wool, nylon and slate), retention was greatest on the riven slate, with attachment up to 100% at water flow of 12.6±0.2 cm second-1, although this was not statistically greater than on wool or nylon. Across all substrates preconditioning by immersion in seawater for two weeks significantly compromised juvenile retention at higher water velocities. Based on these findings, recommended parameters for maximising juvenile P. maximus attachment and retention in water velocities up to 12.6±0.2cm second-1 include utilising a riven slate type hard substrate, preconditioned for 1 week, with juveniles allowed to attach for 24 hours.