The objective of this data collection was to study the structure of the mesopelagic ecosistem off the Northwestern Portuguese coast which is composed by planktonic and nectonics organisms. Mesopelagic organisms live during the day concentrating in big numbers in deeper depths while during the night migrates towards the upper layers. The data was collected aiming to study the composition and the Dynamics of the mesopelagic community and their trophic relations with the fish in the area. Data was collected with an Isaac-Kidd midwater trawl in May 1998 and 2000 and February 1999, together with environmental parameters as temperature, salinity, etc. The results showed a strong temporal variability of the abundance of the diferent taxonomic groups in study. The analysis of the results lead to the conclusion that, being the sampling locations and methods used were similar for the studied period, the big variability in the abundances and taxa have been found must be related mainly to the season of the year in which the sampling was performed (winter season for 1999 and late spring for 1998 and 2000).