During intraspecific interactions involving males, females and subadults, acoustic as well as visual signals were observed in the diurnal frog Phrynobatrachus krefftii. Strikingly, interactions between adult males are highly dominated by inflations of the bright yellow subgular vocal sac without sound production. We studied the signaling behavior in males of P. krefftii at the Amani Nature Reserve, Usambara mountains, Tanzania, from November 2001 to March 2002. Under nonmanipulated conditions, we registered 641 male-male interactions involving 31 individuals during 323 hours of observation. Most (496 or 77 % ) inflations of the vocal sac were purely visual; the remainder (145 or 23%) of the signals were bimodal, and accompanied by sound production. To test whether exclusive visual signaling can be evoked under experimental conditions, we introduced tethered males into the visual field of 28 residents, registering the elicited responses over 10 minutes after the introduction. Unimodal (i.e., exclusive visual) signals (825 out of 1106 responses) dominated over vocal-sac inflations accompanied with sound production. In seven selected focal males, the rate of exclusive visual signaling (0.57 signals/min), however, was more frequent than under nonmanipulated conditions (0.13 signals/min). Phrynobatrachus krefftii is the first species of anuran amphibians reported to perform nonaudible vocal-sac inflations during intraspecific male-male signaling behavior.