Sciaenid fishes are well known for their ability to make sound. These sounds are produced by the interaction between the sonic muscles and swim bladder, and are often associated with disturbance or reproduction. From experiments conducted in captivity and observations made in the field, I characterized another acoustic behavior of Atlantic Croaker (Micropogonias undulatus), termed “knock” calling, and explored hypotheses regarding its function. Knock calls consisted of one to six transient pulses, with two pulses being most common. Mean call duration was 97 msec (SD  =  56, 95% C.I.  =  88–106) msec. Dominant frequencies varied inversely with fish size. Knock calling occurred at all times of day, but calling rates peaked at night for Atlantic Croaker stocked in a research pond. In the pond, per capita calling rate (calls/fish/min) was negatively correlated with the stocking density of Atlantic Croaker. In the field, overall calling rates (calls/min) were positively correlated with Atlantic Croaker density. Knock calling was not associated with either distress or reproductive behaviors. Knock calls differed from disturbance calls with regard to the number of pulses and the temporal spacing between pulses. In captivity, knock calls were recorded when adult and juvenile fish were stocked together, as well as when only juvenile fish were stocked. This study suggests that Atlantic Croaker may use acoustic communication more extensively than believed previously. Further, it reveals a discrepancy between the dominant frequencies produced by juvenile Atlantic Croaker and the known frequency range of this species' hearing, prompting questions about possible changes in frequency sensitivity with ontogeny, the ecological costs of sound production, and juvenile behavior.