PANOW is a software dedicated to the search of rare words in biological sequences (over- or under-represented words in DNA or protein sequences). Using recent results on occurrence times of a string of symbols in a stochastic process with mixing properties (such as Markov chains), it computes point by point error between the law of the number of occurrences of a word and the Poisson approximation of this law. Then, it gives a limit number of occurrences from which a word can be considered as a rare word. PANOW is written in ANSI Cplusplus and developed on x86 GNU/Linux systems with GCC 3.4, and successfully tested with GCC latest versions on Sun and Apple Mac OSX systems. It relies on seqpp library. Compilation and installation are compliant with the GNU standard procedure. PANOW is licensed under the GNU General Public License (GNU GPL).