The microstructure of silk dyed with acid dye was compared with that of undyed, scoured silk by means of x-ray analysis. The four specimens were scoured or dyed with acid dye under varying circumstances. Spacing in all specimens was not essentially altered by treatment in hydrochloric acid solution and dyeing with acid dye in either the absence or presence of sodium n-dodecylsulfate (SDS). The evidence suggested that the take-up of dye occurred in only the amorphous region. The effect of HCl solution on the silk is discussed. The crystallinity and orientation of silk dyed with acid dye in both the absence and presence of SDS is also discussed.