Light is one of the major environmental factors that controls plant development, through a process known as photomorphogenesis. Plants perceive light via photoreceptors, and the information used to direct a myriad of developmental responses. Analysis of mutants defective in photomorphogenic responses elucidates the complex interactions between light and plants. Previous genetic screens have yielded a class of mutants which exhibit exaggerated responses to ambient light, designated shl (seedling hyper-responsive to light). The following work encompasses the identification of five new shl mutants, a detailed examination of one of these mutants (shl7), and of the SHL7 gene. The mutants were isolated in a low-white light screen of seedlings derived from T-DNA mutagenesis. Each of the mutants exhibits a heritable hyper-responsive phenotype in low-white light, but displays minimal effects in darkness. For each, a putative site of T-DNA insertion has been located. In addition to a low-white light phenotype, the shl7 mutant exhibits a mild hyper-responsive phenotype to 670 nm red and 735 nm far-red light, but significant hyper-responses to 420 nm blue light. SHL7 encodes a small, unique, and previously undescribed protein annotated as At4g04925. GFP protein fusion analysis indicates that the protein is localized to mitochondria.