Lymphoid nodules and diffuse lymphocytic infiltration are common in turtle urinary bladders. Nodules, usually located immediately beneath the epithelial layer, were found in 15 of 29 different hemibladders surveyed. A whole cross-section of each hemibladder was examined in the survey. The nodules consisted of aggregates of dense lymphocytes without germinal centers. Erosion of the epithelial layer over the nodule was found in approximately onethird of the cases. Scattered lymphocytes were abundant within the epithelial layer and were found in smears from the mucosal surface. Pyroninophilic cells resembling plasma cells were found in all regions of the bladder. Electron micrographs showed lymphocytes and plasma cells in the epithelial layer above the basement membrane.In the course of a study of the morphology of the urinary bladder of the water turtle, we became impressed with the abundance of lymphoid cells in this tissue. Both lymphoid nodules and diffuse lymphocytic infiltration were common. Previous descriptions of the turtle bladder have not mentioned this finding (LeFevre et al., '71; Rosen, '70a,b). Since knowledge of the nature and extent of lymphoid activity in the turtle bladder is necessary for a full understanding of the functions of this tissue, we investigated the lymphoid cells of the bladder in some detail. The findings and their implications are the topics of the present paper.A preliminary report of this work has appeared elsewhere (LeFevre et al., '72).Turtles (Pseudemys scripta) were obtained from Mogul-Ed Co. (Oshkosh, Wisc.) and were kept in tanks with access to fresh running water until used. The urinary bladder is bilobed and after removal from the turtle was divided into hemibladders. Parasitized and abnormal bladders were discarded. The hemibladders were rinsed, fixed in 10% formalin containing 0.24 M sodium acetate and embedded in paraffin. Sections of whole hemi- MATERIALS AND METHODSANAT. REC., 176: 111-120.bladders were stained with hematoxylin and eosin. Tissue to be examined for pyroninophilic cells was fixed in fresh Carnoy's solution and stained with methyl green-pyronin (Elias, '69 ) .Smears were made by touching the unbroken mucosal surface of a fresh hemibladder to a glass slide and staining with Wright-Giemsa.Small pieces of tissue were fixed in 3% glutaraldehyde in 0.1 M phosphate buffer (pH 7.2) and embedded in Epon. Sections were cut at 0.5 with a glass knife and stained with 1% methylene blue. Sections for electron microscopy were fixed in 3% glutaraldehyde and postfixed in 1% OsO+ both in 0.1 M phosphate buffer (pH 7.2). The sections were embedded in Epon and stained with uranyl acetate. Electron micrographs were taken with a Philips EM 300. OBSERVATIONS Lymphoid nodulesA survey of three groups of turtles obtained in different months was made to determine the frequency of lymphoid nodules in turtle bladders. Random sections of 29 different hemibladders were prepared and the whole circumference of each was ex-