Five species of Alexandrium (A. affine, A. fraterculus, A. leei, A. pseudogonyaulax, and A. tamiyavanichii) are commonly found in Vietnamese waters. They were distinguished based on their apical pore complex (A.P.C), precingular first plate (1′), ventral pore (Vp), and sulcal platelets. A genetic analysis was conducted using nuclear rDNA sequences of ITS and LSU (D1–D3, D8–D10). The growth rates of A. fraterculus, A. leei, A. tamiyavanichii, and A. pseudogonyaulax were quite similar. Specifically, these four species had the highest growth rates at two temperature levels of 24 ◦C and 27 ◦C, at salinities ranging from 25 psu to 35 psu. Furthermore, these species were able to adapt to a low salinity of 20 psu at temperatures from 18 ◦C to 27 ◦C. No Paralytic Shellfish Toxins (PSTs) were found in the two Alexandrium affine strains, VINVN01-1 and VINVN01-2. The detection limit for PSTs ranged from 0.45 to 15.5 fg cell−1, depending on the molecular response and available biomass.