As part of a programme of comparative measurements of diffusional water permeability (P d ) the red blood cells (RBC) from Little Penguin (Eudyptula minor) were studied. The cell dimensions were measured with light and electron microscopy, and by a newly described non-invasive technique, NMR q-space analysis. In view of its relative novelty for cell biologists, an overview of this technique is presented. The RBC revealed an ellipsoidal shape that is characteristic of avian RBC, with axis lengths (''diameters'') estimated to be: a=16.0 μm; b=9.6 μm; c=5.0 μm. The values of P d were: 2.0x10 - 3 cm s - 1 at 5 o C, 3.3x10 - 3 cm s - 1 at 10 o C, 4.6x10 - 3 cm s - 1 at 15 o C and ~5.4x10 - 3 cm s - 1 at 20, 25, 30, 37 and 42 o C.There was a lack of inhibition of water permeability by p-chloromercuribenzensulfonate (PCMBS), the well-known inhibitor of RBC aquaporin. It was notable that in the temperature range 5-20 o C the NMR parameters, and hence the permeability, varied linearly as is found for other species, but at temperatures higher than 20 o C there was no temperature-dependence of P d . Consequently, there was an obvious break at ~20 o C in the Arrhenius plot, of the mean residence life time of water inside the cells, 1/T e , versus temperature. For temperatures less than 20 o C the activation energy E a , d was 45.6+/-6.6 kJ/mol. For temperatures higher than 25 o C E a , d was zero. The lack of inhibition of water permeability by PCMBS and the very high value of E a , d for diffusive water exchange suggests that the water permeation occurs primarily via the membrane bilayer per se, i.e., there is no aquaporin in Little Penguin RBC. The discontinuity at ~20 o C in the Arrhenius plot is an interesting finding, not seen before in other species, and we suggest that it reflects a phase transition in the membrane lipids.