The phenomenon of crowding of ciliates in rest areas has been describe
d previously [23]: these organisms tend to collect on the bottom of an
experimental apparatus under objects interfering with the water-air i
nterface. The behaviour of Oxytricha bifaria has been analyzed as a ba
sic element possibly accounting for the effect itself: to monitor the
phenomenon properly a well-defined time (1 h 15 min) was chosen during
the formation of the overcrowded population; in addition, well-define
d areas were TV recorded, the populations within them studied and thei
r behaviour analyzed and compared. The results obtained are the follow
ing: (a) the number of idle cells is significantly larger in the Shelt
er-Area (S) than in the Open (O); (b) there is a net inward flow of ox
ytrichas, constantly enhancing the cell density in S; (c) the ethogram
of oxytrichas creeping into S is quite similar to that of the populat
ions in O: only the creeping velocity in S is smaller than in O; (d) t
he frequency of Side Stepping Reactions performed by O. bifaria at the
level of the border between S and O is 5-6 times higher for oxytricha
s creeping outwards than for those entering S. The combined effect of
these phenomena well accounts for the formation of the crowding condit
ions.