This paper describes preliminary experiments documenting significant
variations in word usage patterns within topical sublanguages.  As
some phrases have very different collocational patterns than their
constituent words, we look beyond occurrences of individual words, to
consider word phrases.  The mutual information statistic is used to
measure the information content of phrases beyond that of their
constituent words.  We find that specialized topic areas give rise to
phrases with very descriptive constituents which are then ``exported"
into general vocabulary.  These phrases are also much more informative
as word pairs {\em outside} the topic area than within it.  Further,
we find evidence of an intriguing ``self-similar" regularity in this
exporting relation across different hierarchical levels of topical
areas.