Cecropin A is a naturally occurring linear antimicrobial peptide produced by insect larvae; it has potent antimicrobial activity and comprises what appears to be the sole host defense against bacterial infection in these organisms. As part of an array of biophysical investigations into the mechanism by which these peptides accomplish their antimicrobial effect, we are characterizing the rotational activity of the single Trp residue in both free and membrane-bound forms of cecropin A using time-resolved anisotropy measurements of the Trp side chain fluorescence.