Manmade water systems such as cooling towers, potable water systems etc. serve as amplifiers for Legionella pneumophila and non-legionella heterotrophic bacteria (NLHB) by providing suitable conditions for growth and multiplication. In this study we investigated the activity of Pseudomonas and Aeromonas strains, which were isolated from water systems of buildings in Istanbul city, on the growth of L. pneumophila serogroup 1 in vitro. It was found that while the cultures of Pseudomonas species (100%) had a stimulating effect on the growth of L. pneumophila, the cultures of Aeromonas species (61.5%) generally inhibited the growth of L. pneumophila on selective agar medium. Also, it was detected that cell-free supernatants (CFSs) of both Aeromonas, except N24 strain, and Pseudomonas species were not capable of inhibiting the growth of L. pneumophila serogroup 1 strains on buffered charcoal yeast extract agar (BCYEA) plates. However, some of these substances were able to stimulate the growth of L. pneumophila on BCYEA without L-Cysteine (UNBCYEA). It can be thought that the growth of L. pneumophila serogroup 1 could be controlled by the NLHB such as Pseudomonas and Aeromonas that live normally in the same water body. The level of the NLHB can change time to time in water systems and this situation can affect outbreaks of Legionnaires disease.