Towards the latter end of last March, the citizens of Sydney were astonished and alarmed by the sudden discolouration of the water in Port Jackson. The water in the harbour in many places presented the appearance of blood, and the Board of Health immediately requested Mr. W. M. Hamlet, the Government Analyst, to report on the matter. He found that the red colour was due to the presence of a minute organism, which he thought might be the Englena sanguine, Ehrenberg. Immediately after the publication of this report, quite a number of people gave their views of this somewhat mysterious discolouration. It was suggested that it was due to zoospores of some marine Algae, to the Trichodesmium which discolours the Red Sea; and to the young of Medusae; whilst others maintained that it was caused by blood and other refuse turned into the harbour from the abattoirs.