Factors affecting epilithon biomass were investigated at 3 sites along a temporary karst stream system on the Mediterranean island of Majorca. Two sites were on the mainstream axis (downstream site [T5], upstream site [T4]) and one site was in a spring-fed tributary (GB). The degree of abiotic control of periphyton biomass was examined at the 3 sites. Mean periphyton biomass during a surficial flow period (as chlorophyll a [chl a] and ash-free dry mass [AFDM]) was nearly 1 order of magnitude higher at T5 than at the other 2 sites. AFDM ranged from low values at GB to intermediate and very high levels at T4 and T5, respectively. Spatial variation in periphyton chl a was related to the coarser, and more stable, karstic substrata and higher discharge at T5 than at GB or T4. A significant proportion of the temporal variation in periphyton chl a at T4 and T5 was related to abiotic factors, but abiotic factors did not explain the temporal pattern at GB. A short-term field experiment was conducted at GB to determine the role of the dominant grazer, the glossosomatid caddisfly Agapetus quadratus, in regulating periphyton biomass. This experiment revealed that densities of A. quadratus may have controlled periphyton biomass at GB (where grazer densities were highest) because the relationship between A. quadratus density and chl a followed a negative exponential function. The grazer prevented an increase in chl a at intermediate and high densities (>4000 ind./m2), but no effect was found at lower grazer densities. Overall, the levels of calcareous deposition over substratum surfaces seemed to influence colonization by grazers, which, in turn, may have influenced periphyton accrual.