To assess and study the heterogeneity of delta(13)C values for seep microorganisms of the Eel River Basin, we studied two principally different sample sets: sediments from push cores and artificial surfaces colonized over a 14 month in situ incubation. In a single sediment core, the delta(13)C compositions of methane seep-associated microorganisms were measured and the relative activity of several metabolisms was determined using radiotracers. We observed a large range of archaeal delta(13)C values (> 50 per thousand) in this microbial community. The delta(13)C of ANME-1 rods ranged from -24 per thousand to -87 per thousand. The delta(13)C of ANME-2 sarcina ranged from -18 per thousand to -75 per thousand. Initial measurements of shell aggregates were as heavy as -19.5 per thousand with none observed to be lighter than -57 per thousand. Subsequent measurements on shell aggregates trended lighter reaching values as (13)C-depleted as -73 per thousand. The observed isotopic trends found for mixed aggregates were similar to those found for shell aggregates in that the initial measurements were often enriched and the subsequent analyses were more (13)C-depleted (with values as light as -56 per thousand). The isotopic heterogeneity and trends observed within taxonomic groups suggest that ANME-1 and ANME-2 sarcina are capable of both methanogenesis and methanotrophy. In situ microbial growth was investigated by incubating a series of slides and silicon (Si) wafers for 14 months in seep sediment. The experiment showed ubiquitous growth of bacterial filaments (mean delta(13)C = -38 +/- 3 per thousand), suggesting that this bacterial morphotype was capable of rapid colonization and growth.