Publication Rights Reserved This paper is to be presented at the Seventh Annual Meeting of Rocky Mountain Petroleum Engineers of AIME May 25–26, 1961, in Farmington, N. M., and is considered the property of the Society of Petroleum Engineers. Permission to publish is hereby restricted to an abstract of not more than 300 words, with no illustrations, unless the paper is specifically released to the press by the Editor of JOURNAL OF PETROLEUM TECHNOLOGY or the Executive Secretary. Such abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in JOURNAL OF PETROLEUM TECHNOLOGY or SOCIETY OF PETROLEUM ENGINEERS JOURNAL is granted on request, providing proper credit is given that publication and the original presentation of the paper. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and considered for publication in one of the two SPE magazines with the paper. Abstract In recent years the problem of biological control in secondary recovery has received a considerable amount of attention. In many cases, the success of a waterflood has depended upon the ability of the operator to control microbial growth. The waterflood operator is faced with two basic microbiological problems:bacterial corrosion andmicrobial plugging of filters, flow lines, and water injection wells. The waterflood operator must first establish that his problem is biological by a complete microbiological examination of the waterflood system. After finding that such a problem exists and determining the type of organism that is causing the problem, the operator is in a position to evaluate various chemicals for microbial control. In most instances, nitrogen containing compounds such as quaternaries, amines, diamines, and imidazolines are effective in fresh water systems. The materials have the added qualities of being corrosion inhibitors and surface active agents. In general, the nitrogen containing chemicals are not effective in hard water systems. Chlorinated phenol type chemicals have proven to be effective in hard water brines. Introduction Microorganisms in secondary recovery waters are capable of creating problems which, if not controlled, could decrease operating efficiency and; reduce water injection rates. Waterflood operators must know how to detect these problems and how to combat them.