AbstractPhase microscopic observations of 7000 microscopic fields from 50 sediment samples from shallow bays near Port Aransas, Texas suggest that living, (smaller) benthic microorganisms such as diatoms, flagellates, ciliates and blue‐green algae do not support an attached bacterial population and apparently have some protective mechanism against bacterial attack. It is not suggested that larger unicellular organisms secrete large amounts of bactericidal material because motile and nonmotile bacteria were intimately associated with the larger organisms and were often pushed out of the way as the larger organisms moved through the sediment. However, it is possible that certain types of bacteria might be inhibited, which phenomenon could not be detected during the visual observations. It has been postulated that bacteria and unicellular algae may have a similar Zeta or other electrokinetic potentia and are thus repelled during the associations in natural environment