The qualitative and quantitative monthly composition and the annual temporal changes in a shallow (3·3–6·3 m) community of Crustacea: Decapoda associated with soft bottoms (very fine–muddy sand) and the seaweed Caulerpa prolifera in the south of Spain (Valdelagrana Beach in the Bay of Cadiz) has been studied for two years from February 1994 to January 1996. This community showed a similar pattern in both analysed cycles and could be characterized by a change or succession in the abundances of two groups of species: in autumn–winter Hippolyte inermis and Hippolyte varians (species associated with seaweeds); and in spring–summer: Philocheras monacanthus, Diogenes pugilator (characteristic of sandy bottoms) and Macropodia parva. The results of the ordination analysis of monthly samples is related to the absence or abundance of these dominant species although it does not define the two seasonal groups. The analysis of monthly richness, equitability, diversity and k-dominance curves, during the two years, showed a stable and rich community (with more than 6100 specimens caught belonging to 49 species) despite the human influence and the proximity of the Guadalete River.