One of the most striking properties of the pigmented bacterium Serratia marcescens is the phenomenon of color instability. Different strains differ greatly both in color and in the frequency with which they produce color variants. Within a strain, the variations occur at constant rates and are reversible. The high degree of genetic instability is reminiscent of DNA rearrangements such as transpositions, deletions or inversions which are found to occur during various biological processes. During an investigation of this phenomenon Dr. Harshey observed that variation of a 39 kilodalton protein in S. marcescens strain 274, is closely associated with color variation. Using antibodies to this protein she identified it as being the bacterial flagellin. Neither color nor flagellin are produced at temperatures above 37 oC. Thus, pigment synthesis and flagellin synthesis appear to be co-regulated. We propose to isolate and characterize the gene(s) for Serratia flagellin.