When thylakoid preparations of the filamentous blue-green alga Oscillatoria chalybea are exposed to short (2 or 8 μs) saturating light flashes, the oxygen evolution pattern can be distinguished in several respects from the one usually observed in Chlorella. Thus, it appears that a substantial electrochemical signal is already seen under the first flash with maximal flash yield always occurring under the fourth flash. This refers to dark adapted preparations (up to 60 min). Fitting of such an experimental sequence in the 4-state Kok model yields an S-state population consisting of 36-41% S 0 , 40-49% S 1 , 1-10% S 2 and up to 13% S 3 . No abnormality under the first flash is seen in such preparations. Characteristic for sequences with Oscillatoria preparations is a high level of misses which are in the region of 25 per cent, whereas double hits do not seem to play a substantial role in the damping of such sequences. The existence of metastable S 3 , anyway inconsistent with the coherent Kok model, is not confirmed by mass spectrometry. No 18 O 2 seems to be evolved under the first flash from Oscillatoria thylakoids suspended in 50% H 2 18 O. although, when judged from the absolute amperometric signal amplitude, mass spectrometric detection of O 2 should have been possible. With the same method we are fully able to detect 18 O 2 under the second flash in Chlorella vulgaris. In Chlorella this is true for experimental conditions in which the amperometric signal amplitude under the second flash is even smaller than those under the first or second flash in Oscillatoria. The attempt to correlate the amperometrie signal observed under the first flash with a photoinhibition of respiration in our prokaryotic organism was not successful. However, the attempt to incorporate the phenomenon in the coherent Kok model shows that the Oscillatoria sequence fully resembles those with Chlorella, if the first flash signal and 40-50% of the signal observed under the second flash is simply removed from the sequence. The remaining sequence exhibits the usual properties known from Chlorella or higher plant chloroplasts. If one assumes contribution of the reduced state S -1 to the dark population of S-states, a fit in the five rank Kok model yields correct adjustments with a S-state distribution of 6-20% S -1 , 31-40% S 0 , 49-54% S 1 , 0% S 2 and 0% S 3 which would be fully consistent with the Kok model and corresponds to the distribution observed with Chlorella or higher plant chloroplasts. The question what the first electrochemical signal is due to remains unanswered.