Effect of chromate ions on the culture of a marine diatom Phaeodactylum tricornutum was studied using an M-PEA-2 fluorimeter, which carries out simultaneous measurement of fluorescence induction and redox transformations of the P700 pigment within a millisecond range. Chromate ions were shown to inhibit electron transport in PS II and decrease the rate of QА reduction. This results in decreased values of the quantum yield of electron transport in PS II (ϕEo) and performance index (PI ABS), lower rates of P700 reduction, and increased energy (DI0/RC) and ΔpH-dependent nonphotochemical quenching (q E ). Emergence of the slow component of P700 reduction was observed, indicating the activation of cyclic transport in the presence of chromate. Performance index (PI ABS), which was the most sensitive parameter, may be recommended for detection of chromate ions at early stages of their toxic action. The fluorescence parameter F O is promising application in biotesting to assess the algal growth rates.