To uncover components of the mechanism that adjusts the rate of leaf starch degradation to the length of the night, we devised a screen for mutant Arabidopsis thaliana plants in which starch reserves are prematurely exhausted. The mutation in one such mutant, named early starvation 1 or esv1, eliminates a previously-uncharacterized protein. Starch in mutant leaves is degraded rapidly and in a non-linear fashion, so that reserves are exhausted 2 h prior to dawn. The ESV1 protein and a similar uncharacterized A. thaliana protein (named Like ESV1, or LESV) are located in the chloroplast stroma and also bound into starch granules. The region of highest similarity between the two proteins contains a series of near-repeated motifs rich in tryptophan. Both proteins are conserved throughout starch-synthesizing organisms, from angiosperms and monocots to green algae. Analysis of transgenic plants lacking or over-expressing ESV1 or LESV, and of double mutants lacking ESV1 and another protein necessary for starch degradation, leads us to propose that these proteins function[Ed1] in the organization of the starch granule matrix. We argue that their misexpression affects starch degradation indirectly, by altering matrix organization and thus accessibility of starch polymers to starch-degrading enzymes.