This study identifies the most abundant and significant species—from an ecological point of view—in the soft bottom of Cullera Bay (Spain) in order to study the seasonal (summer–winter) variations in the local communities of polychaete annelids and bivalve mollusks. This paper presents the results of the analysis of samples taken during two field campaigns (July 2002 and February 2003) of a series of five campaigns carried out in 2002 and 2003. For these field observations, twelve sampling stations were set up in the bay along three transects. At these stations, macrobenthos was collected using a Ponar grab. Only polychaete annelids and bivalve mollusks were selected from the samples since they made up most of the benthic abundance and biomass respectively. Undisturbed sediment samples were also taken at the same stations to evaluate the influence of environmental factors on these benthic communities. The granulometry, organic matter content, total nitrogen and total phosphorous of these samples were studied. Seasonal-abundance diagrams were obtained for the most abundant groups of species (bivalves and polychaetes) in order to study the macrobenthic fauna associated with the various soft-bottom facies of Cullera Bay and their seasonal patterns. The results show that both polychaetes and bivalves reached their highest total density values in the summer. The assemblages were dominated by Donax trunculus, Donax semistriatus, Spisula subtruncata, Dispio uncinata, Prionospio caspersi and Scoletoma impatiens, whereas the most abundant species were Donax semistriatus, Spisula subtruncata, Dispio uncinata and Prionospio caspersi in the summer and Donax trunculus and Scoletoma impatiens in the winter.