A meter is described which measures the volume of water which has passed through a silk net which may be either towed or lowered and raised vertically.

A double release gear and throttling arrangement allows the net to be opened and, later, closed at any desired depths.

The catch of phytoplankton, from a volume of water which has been measured by the meter, may be assessed from its content of yellow-green pigments, which readily dissolve in acetone.

A rapid method of measuring the pigment content of the catch in terms of arbitrary units of plant pigment is described. The arbitrary unit is related to numbers of two species of diatoms.

The irregular distribution of phytoplankton in the sea is considered in connexion with representative sampling with this measuring net.