The coastal lagoon system of El Carmen-Machona-Redonda, located in the coastal plateau South of the Gulf of Mexico, in the State of Tabasco was studied from August 1977 to July 1978. Tempetature, chlorinity and dissolved oxygen were recorded every two months during the study period. The maximum depth of the system is 3.0 m, decreasing toward the edges and the oyster banks: the average depth is 1.0 - 1.30 m. Only a very small amount of freshwater reaches the system; the chlorinity content is additionally increased on account of the very recent opening of an artificial sea mouth. The highest average temperature (30.1ºC) was recorded in august 1977, and the lowest (25.5ºC) was recorded in december of the same year, thus showing m small annual variation. The average chlorinity value of 4.12‰ between the highest and the lowest average monthly values (19.19-15.47‰) makes it a rather homogeneous system throughout the year. Dissolved oxygen average values varied from 4,52 ml/L in February 1978, to 5.92 ml/L in July of the same year. Thus the system represent a typical, tropical marine lagunar system, with small freshwater influence.