Cell wall and plasma membrane fragments (Figs. S23 and S26) may be misdiagnosed as enveloped pleomorphic viruses, novel viruses, or tailless phages. This error is easily explained. Upon bacteriophage lysis, bacteria fall into pieces, many of which are round and have indeed the size of plasmaviruses or cystoviruses (70-80 nm). The latter are characterized by an envelope surrounding an isometric capsid. When superposed over each other, some cellular elements exhibit what seems to be external and internal (Fig. S25) . Cell wall fragments carrying an adsorbed phage tail may be taken at low magnification for tailed phages (Fig. S26) . These membrane fragments are much more than an inconsequential nuisance because phage counts in seawater and freshwater are increasingly often based on fluorescent microscopy. 19 As it happens, bacterial DNA can associate with membrane fragments during lysis 18 and will stain with fluorescent dies. The fluorescent membrane fragments are approximately of the size of phage heads and appear as tiny green dots simulating phages. So far, investigations of phage prevalence in the environment seem to have neglected this potentially serious source of errors. 18 From Bacteriophages (Tables 2 and 3) Bacterial viruses produce a wide variety of abnormal particles ( Table 3) . Nearly all of them have been observed in tailed phages, although the filamentous inoviruses sometimes give rise to doublelength particles. Tailed phages are classified into three families according to tail structure: Myoviridae with contractile tails, Siphoviridae with long, noncontractile tails and Podoviridae with short tails. Nature and frequency of "freaks" or "monsters" vary with the complexity of phages. Some aberrations have a genetic basis, others reflect errors of assembly, and still others result from propagating phages and their hosts on the wrong substrate, e.g., a medium containing amino acid analogs. 10 The subject has been reviewed elsewhere in some detail. 2 Phage T4 and its relatives have a particular propensity to produce aberrant particles. Some sources of error (and Mr Murphy's and the electron microscopist's delight) are: