There is little consensus on the form of the periphyton biomass–macroinvertebrate diversity relationship in streams. One factor that these relationships do not account for is the growth form of primary producers. We (1) examined the periphyton biomass–macroinvertebrate diversity relationship in 24 streams of Cantabria, Spain, in July 2007, and (2) determined whether this relationship was underpinned, and better explained, by specific responses to the growth form of the periphyton community. We hypothesised that macroinvertebrate diversity would be a log-linear function of periphyton biomass and would respond differently to two coarse divisions of the periphytic community; i.e. positively to %cover of non-filamentous algae and negatively to %cover of streaming filamentous algae. There was no relationship between benthic periphyton biomass and macroinvertebrate diversity in these streams but, as predicted, this relationship was underpinned by responses to the growth form of periphyton community. Generally, macroinvertebrate diversity responded positively to %cover of non-filaments and negatively to %cover of streaming filaments, although results were variable. These findings suggest that periphyton biomass–macroinvertebrate diversity relationships in streams can be underpinned by interactions with specific growth forms of periphyton. We suggest that further research is required to develop robust thresholds of %cover of filamentous algae cover that would benefit managers wishing to minimise negative effects of eutrophication on stream communities.