DissertationFlower visiting flies are one of the most important beneficial groups of Diptera because of their profound pollination services to more than 19% of flowering plants. Many beneficial insects such as bees which pollinate more than 51% of the flowering plants are serious declining in the ecosystems. The production of many insect-dependent crops are at high risk. Previous studies indicated that some families of flower visiting flies contain efficient pollinator species (Larson et al., 2001). These species could be of great importance in maintaining ecosystem service and safeguarding the production of many flowering plants including cucurbit crops. However, knowledge on community structures and visitation rates of flower visiting flies associated with cultivated cucurbit crops is limited in Tanzania. A thoroughly understanding of community structures and visitation rates of flower visiting flies is a prerequisite if their potential is to be realized in agriculture. Therefore, this study assessed the community structures and flower visit activities of flower visiting flies associated with cucurbitaceous production systems in the Morogoro region. Experiments were laid out in a full factorial design. Factors were seasons, agroecological zones, flowering weeks and fly species. Sampling of flies visiting cucurbit flowers was carried out using yellow pan traps and a hand net supplemented with observational counts, in ten established cucurbit fields across the mountainous and plateau zone of the Morogoro region from March to July 2020. A total of 7 606 specimens belonged to 22 genera and 8 families of flower vising flies were collected during the study period. Of which 77.58% of all specimens were collected from mountainous zone and the remaining 22.42% were from the plateau zone. Among the hoverfly species examined, Eristalinus megacephalus Rossi, Mesembrius caffer Loewiii and Toxomerus floralis Macquart showed significant variation in visitation rate, foraging time and abundance across the two agroecological zones, season and sampling weeks (P<0.05). The abundance of these species fluctuated significantly across the two agroecological zones between the wet and dry season (P<0.05). On other hand, both Shannon, Simpson and Margalef indexes placed the mountainous zone as the most abundant and species rich zone in term of number of species. Species abundance distribution models indicated hierarchically arrangement of flower visiting fly’s communities within cucurbit crops. All fields were highly similar as most of the species were shared between fields within each zone. Agroecosystems at different altitudes have different community structures and species within these ecosystems differ in floral visitation rates. The obtained data so far seem to suggest that hoverfly species should be considered as an important pollinators of cucurbit crops. We recommend that a detailed study of pollination efficiency, floral preference and diurnal activities of hoverfly species should therefore be considered a high priority.