An upper bound B 1 is given for the information capacity of the Poisson channel with marginally stationary random noise intensity and causal feedback. The capacity is shown to converge to B 1 in the limit of longer and longer communication intervals for a class of random noise intensities including the case in which the noise intensity is nontime-varying. An upper bound B 1 on the capacity is also established for the Poisson channel with marginally stationary noise and no feedback. In this case

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... also, for a class of random noise intensities including nontime-varying noise intensity, B. 1 is found to be the capacity of the channel without feedback in the limit of longer and longer communication intervals. The fractional difference between B 1 and B 1 is considered as a means to quantify the improvement afforded by feedback. Also, certain nonstandard encoder constraints are addressed and the importance of the encoder intensity peak constraint to the channel capacity problem is explored.