Pilated Neisseria gonorrhoeae of colony type 1 (TI) and non-pilated bacteria of colony type 4 (T4) were observed by transmission (TEM) and scanning electron microscopy (SEM). No pili were observed on T4 gonococci, but two types of pili-straight, type a, and bent, type b-were seen on TI by TEM. When incubated with human sperm and examined by either TEM or SEM, Ti gonococci were seen to attach by individual pili, by several pili wound together as a rope, or by direct contact. Gonococci from T4 colonies attached only by direct contact. Treatment with typsin (I mg/ml) damaged or removed pili from gonococci. After incubation with trypsin, attachment of pilated gonococci to sperm was decreased significantly, but such treatment did not affect attachment of non-pilated gonococci. Incubation of gonococci from either colony type in 0 1 mmol/l ferric nitrate, followed by incubation with sperm, significantly increased attachment of only T4 bacteria. No pili were seen on T4 gonococci treated with ferric ntrate; thus, it appears that factors other than pili alone are concerned in attachment of these gonococci to sperm.