Earlier studies on bacteria and phytoplankton in the Gulf of Cariaco, Venezuela, have been done separately. In this paper, population variations of these planktonic groups and their possible relations were studied from June 1979 to June 1980. Three stations were fixed in the Gulf of Cariaco, Venezuela. Stations 1 and 2 were visited monthly. In situ temperature (°C), salinity (1 x 10-3 ), dissolved oxygen (ml O2. l-1) phytoplanktonic biomass (mg clorof.a.m-3) and bacterial population density (u.f.c.ml-1) were sampled every ten meters from surface to 50 m depth. Station 3 was visited weekly from February to July were sampled every ten meters from surface to 50 m depth. Station 3 was visited weekly from February to July 1980. Samples were taken every five meters from surface to 20 m depth. Temperature profile was similar in the three stations, 19.45-28.10 °C, the highest temperatures were recorded during the stratification period (July-November 1979). The vertical salinity distribution showed differences among stations 1 and 2. The difference is caused by the influence of the discharge of Manzanares River on station 1. Values found were in the range 36.18 x 10-3 to 36.80 x 10-3. Even though, station 1 was influenced by the Manzanares river, there were no significant differences (0.05) between stations 1 and 2 with respect to seawater density (g.ml-1). However, oxygen saturation percentaje showed significant differences, (0.05) between stations 1 and 2. The level of dissolved oxygen exhibited a greater variation (0.06-5.40 ml O2.l-1) in station 3 than in the other two. Phytoplanktonic biomass was higher in stations 2 and 3 (0.40-2.5 mg clorof. a. m-3) than in station 1. Relatively low biomass values (from non-detectable to 1.28 mg clorof. a. m-3) were measured during the calm period. Significant differences (0.05) between stations 1 and 2 in this parameters were found with respect to time but not with respect to depth. Bacterial population density showed significant differences (0.05) between stations 1 and 2 with respect to depth, but not with respect to time. Again, station 1 revealed influence of the Manzanare river, while station 2 showed enrichment caused by upwelling. A linear regression applied to phytoplanktonic biomass and heterotrophic bacterial population density showed a low correlation (r=0.20 & 0.18 at stations 1 and 2, respectively) during the period of stagnation. At this time, microflagellates were the dominant phytoplanktonic group. Once, upwelling had started and diatoms became the dominant group, a negative linear relationsship occured (r=-0.89 & -0.86 for stations 1 and 2, respectively). When the phytoplanktonic community was dominated by dinoflagellates, a direct relationship with bacterial population appeared (r = 0.70 & 0.80 for stations 1 and 2, respectively). It appears that seawater density did not have any influence on the distribution of bacterioplankton, but it did affect phytoplankton. A relationship between bacterioplankton and phytoplankton occurred only when diatoms and dinoflagellates were the dominant phytoplanktonic groups.