The Agami Heron (Agamia agami) is one of the least known species of herons. We report observations on an exceptionally large colony in French Guiana. The estimate of approximately 900 nests exceeds by ten fold the largest colony previously reported. Herons arrived at and departed from the colony and fed young only at night. The characteristic “Gr” call, which was recorded and analyzed, was used a contact-call, which ceased on disturbance. Timing of nesting was highly asynchronous. Nests were over water and in deep shade. Adults shaded young from the sun. Birds appeared to forage individually, possibly at long distances from the colony site.