During the breeding season, Lusitanian toadfish males become territorial and emit an advertisement call (the boatwhistle, BW) to attract mates. We test the hypothesis that boatwhistles can convey information on individual quality by relating calling activity and signal parameters with male physical features, namely male total length, gonadosomatic index, sonic muscle index and male condition, measured by the condition factor and muscle lipid content. We recorded 22 males in the Tagus River estuary (Portugal) during the breeding season (2006 and 2007) for up to eight days and for an average of 35h. Single males were kept inside closed artificial nests and groups of 6-8 individuals maintained a normal chorus activity. Acoustic activity varied greatly among subject males. All males produced boatwhistles during the study period but calling rate varied markedly among them. Average calling rate varied from 0.1 to 361.7 BWh-1 (overall mean = 39.9 BWh-1) and the majority of time was spent in silence (72.4% of the recorded hours). Boatwhistle emission rate and the fundamental frequency of boatwhistles (equivalent to the sonic muscle contraction frequency during sound production) were correlated with male condition (somatic and lipid content). Boatwhistle emission rate was also correlated with sonic muscle index, i.e. with sonic muscle hypertrophy. Calling rate variability also decreased with increased sonic muscle and gonadosomatic indexes. These results suggest that only males with higher somatic and reproductive condition and with higher sonic muscle mass were able to sustain elevated acoustic activity and sonic muscle contraction rates during sound production. In conclusion, boatwhistles seem to be able to signal male quality during social interactions.