We investigate the effect of a magnetic field on a bosonic ladder. We show that such a system leads to the one dimensional equivalent of a vortex lattice in a superconductor. We investigate the physical properties of the vortex phase, such as vortex density and vortex correlation functions and show that magnetization has plateaus for some commensurate values of the magnetic field. The lowest plateau corresponds to a true Meissner to vortex transition at a critical field Hc1 that exists although the system has no long range superconducting order. Implications for experimental realizations such as Josephson junction arrays are discussed.PACS numbers: 05.30. Jp, 71.10.Pm, 74.50.+r The effect of a magnetic field on interacting particles is a long standing problem. A spectacular case is provided by type II superconductor, in which the magnetic field is totally expelled below H c1 , whereas a vortex state exists for H > H c1 . This behavior, however, is obtained from the Landau-Ginzburg equation, and it is important to know what happens when interactions and fluctuations have more drastic effects, such as in one dimensional systems. Indeed, in a one dimensional conductor at T = 0, although there is no long range order, superconductivity in the sense of infinite d.c. conductivity can nevertheless be present 1 . For a one dimensional chain, there is no orbital effect, and this question is not relevant. However, a system made of a finite number of coupled chains (a ladder), is still one dimensional (no long range order can exist) but orbital effect of the magnetic field is present, opening the possibility of such a transition.Beyond its own theoretical interest the investigation of the effect of a magnetic field on ladder systems is also of direct experimental relevance, due to the various realizations of such ladders 2,3,4 . Fermionic ladders can be superconducting both from attractive (s-wave) and repulsive interactions (d-wave). In the attractive case, the system is close to standard superconductors where pairs of fermions can hop from one chain to the other leading to a Josephson coupling, provided the applied magnetic field is smaller that the spin gap. The system can thus be described as a bosonic ladder. Josephson junction arrays 5,6 provide also a very direct realization of such a bosonic ladder 7,8 and are thus the prime candidates to observe these effects. This problem of Josephson ladders has been investigated previously in the classical 9 and quantum limit both analytically 10,11 and numerically 12 in the high field limit of half a flux quantum per plaquette for one dimensional situations. For the Josephson two leg ladder, it has been shown 10,11 that a true transition exists between a commensurate and a vortex phase, however the detailed behavior of the vortex phase and the effects of commensurability of the magnetic field remain to be understood.In this paper we investigate the effect of a magnetic field directly on the bosonic two leg ladder. We study the Meissner-vortex transition in this system, and t...