The exchange of matter and energy between aquatic and terrestrial ecosystems is a key feature of floodplain systems.  On the Cosumnes River floodplain (CA), habitat heterogeneity results in the deposition of both periphyton and leaf litter to soil communities.  To better understand the roles of aquatic and terrestrial production in floodplain soil systems, we asked how invertebrate communities vary between different floodplain habitats and whether differences in the amount and type of organic matter control the density and diversity of soil invertebrates.   We sampled 5-100 m²  plots in 3 habitats on the Cosumnes River Preserve floodplain once before, during, and after winter flooding during 2003-2004.  Habitats included meadows with high, and low algal deposition, and cottonwood forest.   Plots were sampled for soil fertility and invertebrates.  Thirty-seven invertebrate taxa were collected from the plots with worms, beetles, centipedes, and spiders being the most common taxa.  Total numbers of invertebrates decreased over time due to the decrease in worm densities in the forest.  Preflood, worm densities were highest in forest plots and lowest in low algae plots; however, postflood densities were highest in low algae plots.  Understanding processes linking flood pulse, habitat heterogeneity and invertebrate communities is important for understanding soil fertility.